Include common (frequently used) .proto files as static files

[bradleyjkemp]

Definitions such as google/longrunning/operations.proto should be baked into the tools so that it's easier to run grpc-dump/etc. by just pointing at your own proto files.

This should be implemented via a custom file loader in the proto parser which uses static files if real ones cannot be found. (real ones should still take priority if they exist)