{Doc} Add local context developer guidelines (#13668)

* {TestSDK} add delete local context file when test is finished

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This reverts commit f51e802980cb4631527ff621a08d6c6172a62148.

* {Doc} add local context developer guidelines

* Apply suggestions from code review

Co-authored-by: Feiyue Yu <iamyfy@163.com>

* update according to review comments

Co-authored-by: Feiyue Yu <iamyfy@163.com>

doc/local_context_guidelines.md

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+The document provides instructions and guidelines on how to board local context feature for parameters.
+
+## Local context attribute
+
+Local context attribute is an abstraction of the definition for local context. We use this class to define how the argument supports local context.
+
+There are 3 properties of local context attribute.
+
+- **name**
+
+`name` is the argument name used in local context. Usually we just use the name which defined in the function signature, for example, `resource_group_name`.
+
+- **scopes**
+
+`scopes` is an array of strings. It defines where this local context value can be referenced. The value could be a command group or a command, for example, `['vm', 'network']`. If one argument can be referenced in all the commands, you can define it as `['all']`. `scopes` is meaningful only when `SET` is in actions.
+    
+- **actions**
+
+`actions` is an array of [`LocalContextAction`](#Local context action). It defines whether set the command parameter value to local context or get value from local context. We can define both `GET` and `SET` action for one argument.
+
+
+## Local context action
+
+Local context action is defined as an enum type. The available values are `GET` and `SET`.
+
+- **GET**
+
+Used for retrieving value from local context.
+
+- **SET**
+
+Used for saving value to local context.
+
+
+## Parameters which support `all` scope by default
+
+We define local context attribute for some command parameters by default. Commands whose function signature has the same argument name will support local context by default.
+
+All these parameters are listed here:
+
+- *resource_group_name*
+```python
+    local_context_attribute=LocalContextAttribute(
+        name='resource_group_name',
+        actions=[LocalContextAction.SET, LocalContextAction.GET],
+        scopes=[ALL]
+    ))
+```
+
+- *vnet_name*
+```python
+    local_context_attribute=LocalContextAttribute(name='vnet_name', actions=[LocalContextAction.GET])
+```
+
+- *subnet*
+```python
+    local_context_attribute=LocalContextAttribute(name='subnet_name', actions=[LocalContextAction.GET])
+```
+
+For example, below is function signature for `az network vnet create`:
+```python
+def create_vnet(cmd, resource_group_name, vnet_name, vnet_prefixes='10.0.0.0/16',
+                subnet_name=None, subnet_prefix=None, dns_servers=None,
+                location=None, tags=None, vm_protection=None, ddos_protection=None,
+                ddos_protection_plan=None):
+```
+As `resource_group_name` is defined in the signature, it will automatically support local context.
+
+
+## Suggestions
+
+- Only enable local context feature for resource dependency parameters.
+- Do not define `GET` action for `name` parameter in `create` and `delete` command.
+- Define `SET` action only in `create` command.
+
+## Examples
+In order to create a webapp, a user needs to prepare an appservice plan first. Previously the user needed to run below commands to complete this scenario:
+```bash
+az group create --name myResourceGroup --location westus
+az appservice plan create -g myResourceGroup --name myPlan
+az webapp create -g myResourceGroup --plan myPlan --name myWebapp
+```
+The user has to input resource group name 3 times and plan name 2 times for this scenario. To reduce these duplicate type in, we need to enable local context feature for these parameters as below:
+
+- `az group create`: *set resource group name*
+> azure-cli/azure/cli/command_modules/resource/_params.py
+```python
+# define SET action for resource group name
+with self.argument_context('group create') as c:
+    c.argument('rg_name',
+                local_context_attribute=LocalContextAttribute(name='resource_group_name',actions=[LocalContextAction.SET], scopes=[ALL]))
+```
+
+- `az appservice plan create`: *get resource group name & set plan name*
+
+> azure-cli/azure/cli/command_modules/appservice/custom.py
+```python
+# get resource group name from local context by define function argument name as `resource_group_name`
+def create_app_service_plan(cmd, resource_group_name, name, is_linux, hyper_v, per_site_scaling=False,
+                            app_service_environment=None, sku='B1', number_of_workers=None, location=None,
+                            tags=None, no_wait=False):
+```
+
+> azure-cli/azure/cli/command_modules/appservice/_params.py
+```python
+# define SET action for plan name
+with self.argument_context('appservice plan create') as c:
+    c.argument('name',
+               local_context_attribute=LocalContextAttribute(name='plan_name', actions=[LocalContextAction.SET], scopes=['appservice', 'webapp', 'functionapp']))
+```
+
+- `az webapp create`: *get resource group name & get plan name & set webapp name*
+
+> azure-cli/azure/cli/command_modules/appservice/custom.py
+```python
+# get resource group name from local context by define function argument name as `resource_group_name`
+def create_webapp(cmd, resource_group_name, name, plan, runtime=None, startup_file=None,  # pylint: disable=too-many-statements,too-many-branches
+                  deployment_container_image_name=None, deployment_source_url=None, deployment_source_branch='master',
+                  deployment_local_git=None, docker_registry_server_password=None, docker_registry_server_user=None,
+                  multicontainer_config_type=None, multicontainer_config_file=None, tags=None,
+                  using_webapp_up=False, language=None):
+```
+
+> azure-cli/azure/cli/command_modules/appservice/_params.py
+```python
+# define SET action for webapp name
+# define GET action for plan name
+with self.argument_context('webapp create') as c:
+    c.argument('name',
+                local_context_attribute=LocalContextAttribute(name='web_name', actions=[LocalContextAction.SET], scopes=['webapp']))
+    c.argument('plan',
+                local_context_attribute=LocalContextAttribute(name='plan_name', actions=[LocalContextAction.GET]))
+```