OAuth 2.0 authentication support

[dahlia]

Add support for OAuth 2.0 authentication in @upyo/smtp transport.

Background

OAuth 2.0 authentication allows users to authenticate with SMTP servers (such as Gmail, Outlook) using OAuth tokens instead of app passwords. This is increasingly required by major email providers for enhanced security.

Currently, OAuth 2.0 authentication is listed as “planned” in the comparison with alternatives.

References

• RFC 6749 — The OAuth 2.0 Authorization Framework
• google-auth-library-nodejs — Google's OAuth 2.0 library (suggested in Elaborate key differences of nodemailer and other packages #17 (comment))
• msal-node — Microsoft Authentication Library (suggested in Elaborate key differences of nodemailer and other packages #17 (comment))

[dougg0k]

Is there a when for this one?

[dahlia]

No concrete timeline yet, but it's on the roadmap. Contributions are welcome!

[dougg0k]

Ideally the lib should include refresh token support, and regenerate the token automatically, or if too complex, have proper instructions and use the oauth2 libraries as optional based on need.