Implement gRPC GetArtifactType

[tarilabs]

resolves #25

Description

Implement gRPC GetArtifactType

How Has This Been Tested?

in one terminal:

make build
make run/server

in another terminal:

python test/python/test_mlmetadata.py 

then running the following python script:

from grpc import insecure_channel
from ml_metadata.proto import metadata_store_service_pb2
from ml_metadata.proto import metadata_store_service_pb2_grpc

def main():
    channel = insecure_channel('localhost:8080')
    store = metadata_store_service_pb2_grpc.MetadataStoreServiceStub(channel)

    request = metadata_store_service_pb2.GetArtifactTypeRequest()
    request.type_name = "SavedModel"

    response = store.GetArtifactType(request)
    print(response)

if __name__ == '__main__':
    main()

Outputs as expected:

$ python test/python/test_getartifacttype.py 
artifact_type {
  id: 2
  name: "SavedModel"
}

Merge criteria:

• The commits and have meaningful messages; the author will squash them after approval or will ask to merge with squash.
• Testing instructions have been added in the PR body (for PRs involving changes that are not immediately obvious).
• The developer has manually tested the changes and verified that the changes work

[tarilabs]

I preferred to take "same approach" and place this on a tx context too, even if this is only a read, for consistency.

[dhirajsb]

Please add the test client code to the current prototype integration test test/python/test_mlmetadata.py script. We can re-use it in a later testframework.

[dhirajsb]

👍

[isinyaaa]

lgtm!

[tarilabs]

no vetos, no missing, all approval; proceeding to merge.