Implement gRPC `ControlApi` and `CallbackClient`

[ilslv]

Synopsis

Implement gRPC ControlApi and CallbackClient defined in #55.

Checklist

• Created PR:
  • [x] In draft mode
  • [x] Name contains Draft: prefix
  • [x] Name contains issue reference
  • [x] Has k:: labels applied
  • [x] Has assignee
• [x] Documentation is updated (if required)
• [x] Tests are updated (if required)
• [x] Changes conform code style
• [x] CHANGELOG entry is added (if required)
• [x] FCM (final commit message) is posted
  • [ ] and approved
• [x] Review is completed and changes are approved
• Before merge:
  • [x] Milestone is set
  • [x] PR's name and description are correct and up-to-date
  • [x] Draft: prefix is removed
  • [x] All temporary labels are removed

[ilslv]

FCM

Provide gRPC `ControlApi` and `CallbackApi` implementations (#56)