Add NIOSSLCustomVerificationCallback for HTTPClient

[L1MeN9Yu]

We're using pin the public key for ssl-pinning, but there is no way to use NIOSSLCustomVerificationCallback in the NIOSSLClientHandler .

[Lukasa]

While supporting only the NIOSSLCustomVerificationCallback is easy, supporting that and the equivalent functionality in Network.framework with a single code path is substantially harder. We should consider whether adding this support necessarily forces the TLS configuration to choose NIOSSL.

[adam-fowler]

@Lukasa There is already a path for choosing NIOSSL while running with NIOTS so wouldn't be a hard to implement, I imagine.

[Lukasa]

The biggest complexity is about mapping TLS configuration.