Allow peer address to be accessible to the server handler function

[jquirke]

Currently, it is not possible to ascertain from an encoded YARPC handler exactly to who the remote peer is.

For example, it would be useful from and audit and debugging point of view to identify which peer is sending malicious, malformed, or otherwise problematic RPCs.

The proposed interface would be to add the string to the transport.Request structure, so an application can be do something like:

    peerAddress := yarpc.CallFromContext(ctx).CallerPeerAddress()

Note, the design was chosen to be a string to be as underlying transport (transport being L3/L4) agnostic as possible.

Current workarounds are tightly coupled to the underlying protocol. For example, one can use https://pkg.go.dev/google.golang.org/grpc/peer#FromContext to obtain the peer's net.Addr in a GRPC handler, but there is no trivial equivalent for TChannel and HTTP.