rustls TlsAcceptor allow using custom ResolvesServerCert on Listener

[hargut]

This PR allows to use a custom dyn ResolvesServerCert for the rustls TlsAcceptor.

The solution enables certificate resolution based on SNI (#594) and provides flexibility for certificate related topics like certificate reloads during runtime (#611).

Additionally some minor cleanups are part of the PR.

[JockeTF]

Thank you! I gave this a spin, and it seems to be working well with Rustls.