Implement Steps service `Finish` API

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

This includes:

• If relevant, cancelling all in-flight step execution
• Deleting all files generated during job execution (log files, file-type variable files, cache and archive files if relevant)
• Removing entry from service active jobs list.