Add OAuth2 (Modern Authentication) Support for SMTP/Email Notifications

[Mmefko]

Why is this needed:

Many email providers are phasing out Basic Authentication (username + password) in favor of Modern Authentication (OAuth2). For example, Microsoft is deprecating Basic Auth for Exchange Online. Without OAuth2 support, users relying on SMTP email notifications may lose functionality when Basic Auth is disabled.

Some initial work appears to have been done (grafana/alerting#355), but this does not currently include support for email notifications.

What would you like to be added:

Implement OAuth2 (Modern Authentication) support for SMTP/Email in addition to the existing Basic Auth mechanism.

Who is this feature for?

All users who rely on email notifications and need to comply with providers requiring OAuth2 authentication.