mgrpc: baseline protobuf example

Set the stage for the example server by including a service definition,
generated code and the protobuild definition file used to generate them.
From here, we can work up to a generated mgrpc server.

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

example/doc.go

@@ -0,0 +1,2 @@
+// Package example demonstrates a lightweight protobuf service.
+package example