api/services: define the introspection API

After several requests for information about platform support,
configuration introspection and feature switches, we now have a solution
that should work in all these use cases. The Introspection API hooks
into the plugin subsystem registration process. During registration,
plugins declare several pieces of information, allowing clients to
discover the cababilities and support that a containerd instance
provides, including whether or not it loaded with an error.

To allow symmetrical error reporting, the `google/rpc.Status` protobuf
definitions have been brought in from the googleapis project.
Unfortunately, we had to generate these in place to match our protobuf
system.

Once we like this design, we can add an implementation to integrate it
directly with the plugin system.

Enjoy!

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

api/services/introspection/v1/doc.go

@@ -0,0 +1 @@
+package introspection