Implement Steps service FollowLogs API

Implement the FollowLogs() API as per the Blueprint.

• Closes #36 (closed)
• Best reviewed commit-at-a-time

This is part of an MR stack:

• Implement Steps service FollowLogs API (!67 - merged) 👈
  • Implement Steps service Status API (!70 - merged)
    • Add step-runner gRPC service Client (!77 - merged)