Add remote and/or local addresses for unix domain socket

[ejona86]

This is split out of #9900, which had two pieces. See it for more details. CC @carl-mastrangelo

Unix domain sockets are strange from a local/remote address perspective, as only one of them generally has a well-defined name (file path). We need to see what other languages do, but at the very least for this to be considered completed we should have one of local/remote for both client and server.

Right now apparently the code is missing both, when using KQueue or Epoll, as Netty is not providing either:

{io.grpc.internal.GrpcAttributes.securityLevel=NONE, io.grpc.Grpc.TRANSPORT_ATTR_REMOTE_ADDR=null, io.grpc.Grpc.TRANSPORT_ATTR_LOCAL_ADDR=null}

We might consider enhancing Netty, but that'd probably take some double-checking in how Linux itself handles this. If we think we can't even add addresses (because we find that Netty did it that way "for a good reason"), we need an attribute of some sort to identify UDS connections, so absolute worst-case would be a SocketAddress that just said "this is a UDS connection" without any other information.

[ejona86]

Netty now may be adding them. netty/netty#13323