Provide a `TLSConfiguration.makeServerWithMTLSConfiguration` or similar to make it less dangerous to configure mTLS with NIO

[weissi]

Configuring mTLS servers with NIOSSL is a little dangerous and weird. Here is what you're meant to do:

        var tlsConfiguration = TLSConfiguration.makeServerConfiguration(
            certificateChain: serverChain
        )
        if requireMTLS {
            tlsConfiguration.trustRoots = caChainToCheckClients
            tlsConfiguration.certificateVerification = .noHostnameVerification // THIS IS IMPORTANT(!!)
        }
        return tlsConfiguration

The crucial bit here is tlsConfiguration.certificateVerification = .noHostnameVerification and this fails open. So if you forget that line, then it defaults to .noVerification which just doesn't do mTLS at all. And .fullVerification is also no good because otherwise it tries to check the client's host name.

So I believe there's only one correct construction and that's the one above. This should be made much easier for the user.

Furthermore, it might make sense to raise from .noValidation to .noHostnameVerification if the user sets trustRoots, why else would they set it?

[Lukasa]

Yeah, these both seem like reasonable requests.

[Lukasa]

For the second part: #543

[Lukasa]

Ok we're backing that out: #544. That turns out to break a bunch of folks. I think this API might be beginning to creak under the weight.