Provide an option to specify base context for RPC connection

[homier]

Rationale

Providing a function to build base context is really useful in situations, when we need to propagate some, let's say, identity data to server handlers. Let's take the following issue as an example:

• "I need to pass the remote peer ID to my underlying handlers, so I could do some basic identification stuff"

The problem is that now the only way to do this is to set it in rpc.Conn and pass the connection to handlers, but this breaks abstraction between transport layer and application layer.
But we could a simple change to have a context factory, so all the necessary connection information could be set in context, that will be passed to handlers later. And it's similar to how the native http.Server interface is built.

Real-life example (using libp2p as network stack):

package handler

import 	(
       "capnproto.org/go/capnp/v3/rpc"
       "github.com/libp2p/go-libp2p/core"
)

type Handler struct {
    ...
}

func (h *Handler) Serve(stream core.Stream) *rpc.Conn {
        remotePeerID := stream.Conn().RemotePeer()

	conn := rpc.NewConn(rpc.NewPackedStreamTransport(stream), &rpc.Options{
		BootstrapClient: h.resolverHandler.Client(),
		BaseContext: func() context.Context {
		       return context.WithValue(context.Background(), "peerID", remotePeerID)
		},
	})

	return conn
}