Implement Service Client Library

Implement a client library to manage the entire lifecycle of a job (Run, Follow*, Finish), including reconnecting to the Follow* APIs when the connection is lost. It should be possible for clients to receive results from the Follow* APIs in a streaming fashion. This module should reside in the common location determined in #30.

See https://docs.gitlab.com/ee/architecture/blueprints/gitlab_steps/runner-integration.html#steps-service-grpc-definition

See !21 (diffs) for working example