Export protobuf sources as filegroup

[lizan]

The source files needed for descriptor generation, at https://github.com/lizan/envoy/blob/aa3aada1fa9e2557e34c1144d91bb2241da84ebf/test/proto/BUILD#L24 in envoyproxy/envoy#1079

[htuch]

@lizan Out of curiosity, who is doing the descriptor generation? I.e. what are you working on with v2 API dependency?
@mattklein123 Can you review/merge staging to master before we do this merge?

[mattklein123]

#78

[lizan]

@htuch the transcoding filter need protobuf descriptors as config, sample:
https://github.com/istio/proxy/blob/master/src/envoy/transcoding/envoy.conf#L37
and it is depending google/api/annotations.proto and google/api/http.proto for the HTTP mapping.

The descriptor is generated from .proto files, so we have a genrule to generate descriptors and load it into tests. Since envoy's googleapis repo are loaded from here, that what this PR is.

Alternatively we can move this dependency loading to envoy repo itself, I'm fine with either way.

[htuch]

@lizan We've switched back to working on master. I changed the base of this PR, but there are now merge conflicts. Can you resolve these and update the PR? Thanks!

[lizan]

@htuch rebased. PTAL.

[htuch]

@lizan Looks like Travis is unhappy, can you take a look?

[lizan]

Seems {cc,py}_proto_library doesn't like filegroup as srcs, revert that part.