refactor: plugin email without auth

[wI2L]

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

Refactor the email plugin to allow using it with SMTP servers that doesn't require authentication.

• What is the current behavior? (You can also link to an open issue here)

Configuration fields smtp_username and smtp_password are mandatory.

• What is the new behavior (if this is a feature change)?

The username and password fields are no longer required, and can be left empty. In this case, the mail.v2 package will simply try to connect to the SMTP server without authentication. See https://github.com/go-mail/mail/blob/v2.3.1/smtp.go#L122

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)

No, current configurations for this plugin will continue to work as expected.

[rbeuque74]

LGTM, useful when using with internal smtp servers that might not require authentication