Allow bypass plain auth tls check

[lublak]

Is your feature request related to a problem? Please describe.

Currently with use a special smtp tunnel without encryption and plain text (we have old hardware).
With the tunnel we secure the connection.
Currently there is not option to bypass it in your library:

https://github.com/wneessen/go-mail/blob/main/smtp/auth_plain.go#L41

Describe the solution you'd like

An option to set plain text without require tls and localhost.

Describe alternatives you've considered

Currenlty i copied the code and use SetSMTPAuthCustom

Additional context

No response

[wneessen]

Thanks for the issue. Interesting use-case. Definetly something to consider.

[wneessen]

PR #344 implements two new SMTPAuthType types: SMTPAuthPlainNoEnc and SMTPAuthLoginNoEnc. These allow for PLAIN and LOGIN auth via unencrypted channels.

[lublak]

@wneessen awesome :) thank you!