Azure Quantum client: Add environment variable to set User-Agent prefix (#805)

* Add environment variable to set User-Agent prefix

* Reuse QuantumJobClientOptions to pass the user-agent prefix

src/Azure/Azure.Quantum.Client.Test/WorkspaceTest.cs

@@ -196,7 +196,7 @@ Tests will be marked as Inconclusive if the pre-reqs are not correctly setup.";
             }
 
             var options = new QuantumJobClientOptions();
-            options.Diagnostics.ApplicationId = "ClientTests";
+            options.Diagnostics.ApplicationId = Environment.GetEnvironmentVariable("AZURE_QUANTUM_NET_APPID") ?? "ClientTests";
 
             var credential = Authentication.CredentialFactory.CreateCredential(Authentication.CredentialType.Default);
 

src/Azure/Azure.Quantum.Client/JobManagement/Workspace.cs

@@ -51,6 +51,8 @@ namespace Microsoft.Azure.Quantum
             // Optional parameters:
             credential ??= CredentialFactory.CreateCredential(CredentialType.Default, subscriptionId);
             options ??= new QuantumJobClientOptions();
+            options.Diagnostics.ApplicationId = options.Diagnostics.ApplicationId
+                                                ?? Environment.GetEnvironmentVariable("AZURE_QUANTUM_NET_APPID");
 
             this.ResourceGroupName = resourceGroupName;
             this.WorkspaceName = workspaceName;