Add functionality into GrpcWorkerChannel to send and receive worker indexing message types

[anandagopal6]

For the worker indexing code path, GrpcWorkerChannel needs to be able to send a metadata request message to the worker after subscribing to the metadata response message from the worker.

Upon receiving metadata from the worker, GrpcWorkerChannel needs to parse the response into FunctionMetadata objects and send these objects to the next step in the pipeline for further validation.

[anandagopal6]

The necessary code has been added into GrpcWorkerChannel to take care of this. I tested the new functionality by mocking inbound worker responses in unit tests and this addition works as expected.