Add TLS support for NIOPosix H2 client

[gjcairo]

Motivation

We currently have a NIOPosix client transport implementation in gRPC v2, but it doesn't support TLS.

Modifications

This PR adds support for TLS in the NIOPosix-backed HTTP/2 implementation of the client transport for gRPC v2.

Result

We now support TLS when using the NIOPosix client transport in gRPC V2.

[glbrntt]

This looks great so far. I left a few comments but the bigger piece of feedback is that we should now be adding some end-to-end tests for this.

[glbrntt]

One small thing to shuffle around but looks good otherwise, thanks Gus!

[glbrntt]

These are expensive to create (IIRC it's of the order 20k allocations!) and at the moment we do it per connection. Can we do it in init instead and then reuse it?

[gjcairo]

While making this change I realised that we're exposing the tls case for TransportSecurity even when NIOSSL cannot be imported. I think this is unsafe, because it means users may be able to add a TLSConfig to their server/client, which then simply will be ignored if the platform the code's being built on doesn't support NIOSSL. If this happens on both the server and the client, it could mean the connection is plaintext and there would be no warnings that the config wasn't used.
I've wrapped the tls case of TransportSecurity in #if canImport(NIOSSL) to avoid this. Let me know if you agree with it or if I'm missing something/think it should be resolved in some other way.

[glbrntt]

LGTM, thanks Gus!