feat(auth): user pool custom endpoint support

[atierian]

Issue #, if available: N/A

Description of changes: Adds custom endpoint support for user pools.

Check points: (check or cross out if not relevant)

• [x] Added new tests to cover change, if needed
• [x] Build succeeds with all target using Swift Package Manager
• [x] All unit tests pass
• [ ] ~All integration tests pass~
• [x] Security oriented best practices and standards are followed (e.g. using input sanitization, principle of least privilege, etc)
• [ ] Documentation update for the change if required --> pending
• [x] PR title conforms to conventional commit style
• [x] If breaking change, documentation/changelog update with migration instructions

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[atierian]

Note for future self: We could/should add an integration test to assert that the custom endpoint flow is supported. I tested this change manually with an Amplify CLI configured user pool in addition to lambda, secrets manager, and cloudfront resources created outside of the CLI. We'd have to add a separate amplifyconfiguration.json and a define a CFN stack to deploy what we need. That manual test + thorough unit testing gives me the confidence that this works as expected, however a full integration test(s) would be a great addition to protect against regressions.