Artifact as a Proto: protos and service
Part of #909, as we move to artifact specific services.
This adds the protos and a grpc service which is added to the
bst-artifact-server, checking whether the files directory is present or actually a directory. It does not check anything deeper than that which is left to the client.
At the moment this doesn't affect any of the rest of buildstream.
Changes proposed in this merge request:
- Add artifact proto
- Add grpc service
- Test grpc service
This merge request, when approved, will close: #965 (closed)