A way to test email configuration

### Description

Provide a way to test the email configurations work, perhaps in UI or CLI.

### Use case/motivation

Airflow provides configurations for how emails should be sent. But after changing the configurations, it’s desirable to have a way to ensure they’re working by sending a test email, since otherwise it means the user is going to miss potentially critical email notifications and discover the error too late.

The recent “Test Connection” addition on Connection entires is pretty awesome. Maybe we could do the same (although I’m not sure where the button should go). If there’s not a suitable UI location (or if the web server is not the correct process to send emails), we can add a CLI subcommand to do this instead.

### Related issues

_No response_

### Are you willing to submit a PR?

- [X] Yes I am willing to submit a PR!

### Code of Conduct

- [X] I agree to follow this project's [Code of Conduct](https://github.com/apache/airflow/blob/main/CODE_OF_CONDUCT.md)

Start by tracing Airflow's email configuration and the existing Connection “Test Connection” flow. Compare possible UI and CLI entry points, then define how a successful test email and configuration failure should be reported. Done means users have a documented way to send a test email and verify the configured delivery path.

Written by the indexing model from the issue text.