Creating a new version for releasing the SMTP Mail changes to Beta

[maigovannon]

This PR is to review the 2.0.39-beta version to get it for Beta release to Maven. This primarily is targeted at releasing the SMTP Mail changes to Beta customers

Note: This PR is not meant to be merged to main but only to be reviewed for correctness and released directly to maven from this branch. Hence only created a draft PR since it will never be ready to merge.

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Release Notes

This release introduces a major feature to the App Engine Mail API: the ability to send emails via an
external SMTP service. This provides a more flexible and robust way to send emails from your App Engine
application.

Mail API: SMTP Support

Key Features:

• SMTP Mail Sending: Implemented the ability to send emails through a configured SMTP server. This
  functionality can be enabled by setting the USE_SMTP_MAIL_SERVICE environment variable.
• Admin Email Support: AdminEmailMessage is now supported when using the SMTP transport. Recipients for
  admin emails are configured via the ADMIN_EMAIL_RECIPIENTS environment variable.
• Error Handling & Resilience: Added specific error handling for common SMTP issues, such as authentication
  and connection errors, which are now surfaced as standard Mail API exceptions.
• Comprehensive Tests: A full suite of unit tests has been added to ensure the reliability and correctness
  of the new SMTP sending functionality under various conditions.

[ludoch]

LGTM for staying in this branch and pushing the beta release