Add an expansion service to Go SDK

This will allow Go SDK to offer it's transforms to other SDKs.

This should support Beam [transform expansion API](https://github.com/apache/beam/blob/master/model/job-management/src/main/proto/beam_expansion_api.proto) and should have support for registering transforms to be offered through the transform expansion API.

Imported from Jira [BEAM-9923](https://issues.apache.org/jira/browse/BEAM-9923). Original Jira may contain additional context.
Reported by: chamikaramj.

Start by reading the Beam transform expansion API definition in beam_expansion_api.proto and the Go SDK structure. Determine the entry point for an expansion service and how transforms can be registered for exposure through the API. Done means the Go SDK provides the service and supports registering transforms for other SDKs.

Written by the indexing model from the issue text.