chore(docs): Add ADR for Google Auth Library approach

docs/adr/0029-using-google-auth-library.md

@@ -0,0 +1,79 @@
+# Using Google Auth Library
+
+- Deciders: Vijay Budhram, Wil Clouser, Stéphanie Ouillon, Frida Kiriakos
+- Date: 2022-02-03
+
+## Context and Problem Statement
+
+To help improve Firefox account adoption we want to give users flexibility when it comes to signing up for an account.
+To do this we plan on adding support for a limited number of clients that will allow them to create a Firefox account from a Google account.
+
+This ADR covers the options we looked at to integrate with Google Auth.
+
+## Decision Drivers
+
+- Future maintenance
+- Ease of integration
+- User experience
+- Security implications
+
+## Considered Options
+
+To add support for Google authentication we have a couple of options
+
+1. Load Google’s [Identity Services framework](https://accounts.google.com/gsi/client) directly on FxA domain to implement the authentication
+2. Load Google’s [Identity Services framework](https://accounts.google.com/gsi/client) on a subdomain of FxA
+3. Implement Google Oauth2 authorization code flow (does not require 3rd party library)
+
+## Decision Outcome
+
+At this current time we decided to go with Option 3 (Implement Google Oauth2 authorization code flow).
+
+While Option 2 seems like the best compromise between user experience and security, it is more complex and could potentially introduce more bugs. In the future, we can try to prototype this approach.
+
+## Pros and Cons of Options
+
+### Load Google’s Identity Services framework directly on FxA domain
+
+Pros:
+
+- Very easy to implement
+- Google will be deprecating [older framework](https://developers.googleblog.com/2021/08/gsi-jsweb-deprecation.html) to support this
+- Library has several new features such as one-tap signin that greatly reduce friction for creating an account
+
+Cons:
+
+- Can not audit library
+- No versioning (we can not see when changes occur)
+- No official or clear channel of communication for any updates
+- Little visibility on what is going on in the code
+- If an attacker exploits a vulnerability in the lib to abuse the flow they might be able to login into an account
+
+### Load Google’s Identity Services framework on a subdomain of FxA
+
+Pros:
+
+- Reduces risk of account being abused since library is loaded on a different domain
+- Library has several new features such as one-tap signin that greatly reduce friction for creating an account
+
+Cons:
+
+- Requires shuffling around our `email-first` page which could be complex and introduce bugs in our code and metrics
+- Can not audit library
+- No versioning (we can not see when changes occur)
+- No official or clear channel of communication for any updates
+- Little visibility on what is going on in the code
+
+### Implement Google OAuth2 authorization code flow
+
+Pros:
+
+- We don't load any third party javascript
+- Google uses oauth and openid standards so the implementation is straightforward
+- We can audit our code and push fixes as needed
+- User experience would be the same as Pocket Google Auth flow
+
+Cons:
+
+- Would not have one-tap signin and user experience would not be as good as using the Google library
+- We can introduce bugs into our code