TUN-8424: Refactor capnp registration server

Move RegistrationServer and RegistrationClient into tunnelrpc module
to properly abstract out the capnp aspects internal to the module only.

tunnelrpc/registration_server.go

@@ -0,0 +1,40 @@
+package tunnelrpc
+
+import (
+	"context"
+	"io"
+
+	"zombiezen.com/go/capnproto2/rpc"
+
+	"github.com/cloudflare/cloudflared/tunnelrpc/pogs"
+)
+
+// RegistrationServer provides a handler interface for a client to provide methods to handle the different types of
+// requests that can be communicated by the stream.
+type RegistrationServer struct {
+	registrationServer pogs.RegistrationServer
+}
+
+func NewRegistrationServer(registrationServer pogs.RegistrationServer) *RegistrationServer {
+	return &RegistrationServer{
+		registrationServer: registrationServer,
+	}
+}
+
+// Serve listens for all RegistrationServer RPCs, including UnregisterConnection until the underlying connection
+// is terminated.
+func (s *RegistrationServer) Serve(ctx context.Context, stream io.ReadWriteCloser) error {
+	transport := SafeTransport(stream)
+	defer transport.Close()
+
+	main := pogs.RegistrationServer_ServerToClient(s.registrationServer)
+	rpcConn := rpc.NewConn(transport, rpc.MainInterface(main.Client))
+	defer rpcConn.Close()
+
+	select {
+	case <-rpcConn.Done():
+		return rpcConn.Wait()
+	case <-ctx.Done():
+		return ctx.Err()
+	}
+}