mgrpc: initial implementation of server

With this change, we define a simple server and client framework to
start generating code against. We define a simple handler system with
back registration into the server definition.

From here, we can start generating code against the handlers.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

client.go

@@ -0,0 +1,51 @@
+package mgrpc
+
+import (
+	"context"
+	"net"
+
+	"github.com/containerd/typeurl"
+)
+
+type Client struct {
+	channel *channel
+}
+
+func NewClient(conn net.Conn) *Client {
+	return &Client{
+		channel: newChannel(conn),
+	}
+}
+
+func (c *Client) Call(ctx context.Context, service, method string, req interface{}) (interface{}, error) {
+	payload, err := typeurl.MarshalAny(req)
+	if err != nil {
+		return nil, err
+	}
+
+	request := Request{
+		Service: service,
+		Method:  method,
+		Payload: payload,
+	}
+
+	if err := c.channel.send(ctx, &request); err != nil {
+		return nil, err
+	}
+
+	var response Response
+
+	if err := c.channel.recv(ctx, &response); err != nil {
+		return nil, err
+	}
+
+	// TODO(stevvooe): Reliance on the typeurl isn't great for bootstrapping
+	// and ease of use. Let's consider a request header frame and body frame as
+	// a better solution. This will allow the caller to set the exact type.
+	rpayload, err := typeurl.UnmarshalAny(response.Payload)
+	if err != nil {
+		return nil, err
+	}
+
+	return rpayload, nil
+}