feat: Add support for custom Lambda function email senders in Auth construct

[vigy02]

Problem

This PR addresses the need for more flexible email sender configuration in the Auth construct, specifically allowing the use of custom Lambda functions for email sending capabilities.

Changes

• Added support for using Lambda functions as email senders in the Auth construct
• Implemented KMS key management for custom email senders
• Updated types and interfaces to accommodate new email sender options
• Added new test case for custom email sender functionality
• Modified existing code to handle Lambda function email senders, including permission setup

Validation

A new test case was added to verify the behavior when a function is provided as an email sender. This test ensures that the custom email sender is properly set up when a Lambda function is provided.

Checklist

• [x] If this PR includes a functional change to the runtime behavior of the code, I have added or updated automated test coverage for this change.