Implement Steps service `FollowLogs` API

Implement the Steps service FollowLogs API as per https://docs.gitlab.com/ee/architecture/blueprints/gitlab_steps/runner-integration.html#steps-service-grpc-definition.

This includes:

• Initiate following of sub-process and step-runner logs as they are emitted.
• The caller should be able to specify a byte-offset from which to start streaming.
• Handling ill-behaved client that disconnect prematurely.

See gitlab!144373 (merged) for dicussion and context.