Control API gRPC callbacks for Member (#61)

[evdokimovs]

Resolves #61

Part of #27

Synopsis

In 0001-control-api RFC exists callback mechanism but currently it doesn't implemented. In this PR will be implemented only on_join and on_leave gRPC callbacks.

Solution

• Add protobuf spec for callback service in medea-control-api-proto
• Extend medea-control-api-mock with gRPC callback server mock
• Add basic structure for the Control API callbacks
• Add on_join and on_leave callbacks sending
• E2E tests for Control API callbacks
• Add callbacks to e2e-demo's Control API UI

Checklist

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

[evdokimovs]

FCM

'on_join' and 'on_leave' gRPC Control API callbacks (#63, #61)

- add Protobuf spec for Callback service into 'medea-control-api-proto' crate
- extend 'medea-control-api-mock' with gRPC callback server mock
- impl 'on_join' and 'on_leave' callbacks sending
- add callbacks to e2e-demo's Control API UI
- add E2E tests for Control API callbacks

Additionally:
- delete 'gen_insert_str_macro!' from tests
- delete temporary gRPC Control API client binary
- 'build.rs' of 'medea-control-api-proto' now generates source code for all Protobuf files from 'grpc' module
- upgrade Firefox to 71.0 version for E2E tests

[tyranron]

The overall implementation feels quite raw and unpolished. You should spend more time on polishing and self-reviewing this.

[tyranron]

Not without corrections, but majorly good 👍