SpringBoot Add SslBundle support to MailSender

Currently, MailProperties doesn't support SslBundle configuration. There are scenarios where a client has a SMTP server with a certificate signed by a local CA and we don't have a simple way to configure MailSender to use a different SSL context, and SslBundles seems to be a great fit here.

This PR adds support to SSL configuration using a SslBundle.

Comment From: ruifigueira

I added some integration tests using a Mailpit testcontainer, both in TLS/SSL mode and using STARTTLS.

Comment From: mhalbritter

Thanks a lot!