Add SMTP authentication support for globally configured relay servers

[adepretis]

Summary

This change adds SMTP authentication support for globally configured relay servers in postal.smtp_relays.

Postal already supports relay entries like:

postal:
  smtp_relays:
    - smtp://hostname:port?ssl_mode=...

With this change, relay URLs can also include SMTP credentials:

postal:
  smtp_relays:
    - smtp://username:password@hostname:port?ssl_mode=...

When credentials are present, Postal connects to the relay using SMTP LOGIN authentication. When they are not present, behavior remains unchanged.

What changed

• Extended relay URL parsing in lib/postal/config_schema.rb to extract:
  • username
  • password
• Extended SMTPClient::Server to carry optional relay credentials.
• Updated relay construction in SMTPSender.smtp_relays to pass parsed credentials through to the SMTP client layer.
• Updated SMTPClient::Endpoint#start_smtp_session to call:
  • start(helo_hostname, username, password, :login) when credentials are configured
  • start(helo_hostname) otherwise
• Updated configuration documentation to mention the authenticated relay URL format.
• Added test coverage for:
  • unauthenticated relay URLs
  • authenticated relay URLs
  • percent-encoded credentials
  • authenticated relay propagation through SMTPSender
  • authenticated SMTP session startup

Backward compatibility

This is fully backward compatible for existing installations using unauthenticated relay URLs.

Notes

• This change is intentionally limited to postal.smtp_relays.
• It does not add authentication support to DB-backed SMTPEndpoint records or the UI.
• Authentication type is fixed to SMTP LOGIN for now.

[adepretis]

@adamcooke any comment?

[adepretis]

Anyone? - @willpower232 maybe?