feat: preparing auth interfaces for partner auth plugins

[somensi-aws]

Issue # (if applicable)

Closes #.

Reason for this change

Split Auth interface and CognitoAuth implementations in preparation for partner Auth plugins

Description of changes

• Split Auth interface and CognitoAuth implementations
• Documented Auth interface
• Implemented auth custom resource deletion for Cognito UserPool and UserPool domain
• Removed CognitoAuth layers
• UserManagement, TenantManagement, Authorizer, Auth-Custom-Resource split per folder
  • Each service code is deployed on its own
  • Standirdized how AWSLambdaPowertools are deployed in Lambda functions
  • Cleanup requirements.txt
  • all function entry points are index.py
• fix: get_user returns 404, "User not found" if user doesn't exist

Description of how you validated changes

Integration tests

Checklist

• [X] My code adheres to the CONTRIBUTING GUIDE

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[suhussai]

@tobuck-aws , @ujwalbukka , would love to get your thoughts here too.

@somensi-aws , I added a couple other things but also saw your note about standardizing in a follow-up PR. If that's the plan, then those can be ignored. Thanks! This is awesome stuff, by the way!