smtp: add LOGIN SASL auth directive

[osnabrugge]

Summary

Add support for a login SASL authentication directive in the SMTP target.

Changes

• Extend the SMTP SASL directive parser to accept login alongside plain
• Use sasl.NewLoginClient(...) when login is selected
• Add a focused unit test covering the LOGIN directive and mechanism name

Why

Some SMTP relays only support the LOGIN authentication mechanism and reject PLAIN. This makes Maddy usable with those upstream relays without requiring downstream patching or custom images.

Validation

• Patch applies cleanly on current upstream
• go test ./internal/target/smtp -run TestSASL_Login_Directive -count=1 -timeout 90s

Notes

This is a narrow change limited to the SMTP target SASL directive path and preserves existing plain behavior.

[foxcpp]

Thanks!

[osnabrugge]

You're more than welcome! This was needed so that Maddy could authenticate to Azure Communication Services