[Feedback] Add example or template for a Lambda Trigger to link existing users

[bjoerndotsol]

Page: /cli/usage/lambda-triggers

Feedback:
Hi,
I have several sign in methods (Google, Apple, Email + Password) and would like to link user accounts to ensure that a user is not created twice.

From the documentation and various github issues, I understood, that you should use a pre-signup trigger to handle these kind of situations but I could not find any template or best practice from AWS even though this seems to me like a pretty standard use case.

Therefore I would appreciate if you could add something related to your documentation.

So far, I found the following resources that I considered as helpful:

• Lambda trigger code examples:
  AWS Cognito: Best practice to handle same user (with same email address) signing in from different identity providers (Google, Facebook)

• The related issue if you link the user for the first time:
  Cognito auth flow fails with “Already found an entry for username Facebook_10155611263153532”

By the way, Firebase has this already in its documentation:
https://firebase.flutter.dev/docs/auth/usage/#linking-user-accounts

Thanks,
Bjoern :)

[siegerts]

Chatted with @sachscode offline and he raised good points that this needs standardized across sections.

Currently, there are external guides that are relied on for most of these questions - we need to 1) roll these into the docs, or 2) standardize and develop content around the "How to" of this.

ref: https://dev.to/dabit3/the-complete-guide-to-user-authentication-with-the-amplify-framework-2inh

/cc @aws-amplify/amplify-sa