Implement RTC stats for native platforms

[evdokimovs]

Synopsis

We need to implement processing of flutter-webrtc's RTC stats on medea-jason side.

Solution

At this moment it should be done by converting RTC stats to the JSON and then parsing them on Jason side the same way as we're doing it for Web.

Checklist

• Created PR:
  • [x] In draft mode
  • [x] Name contains issue reference
  • [x] Has type and k:: labels applied
  • [x] Has assignee
• Before review:
  • [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 or updated
  • [x] Draft mode is removed
• [x] Review is completed and changes are approved
  • [x] FCM (final commit message) is approved
• Before merge:
  • [x] Milestone is set
  • [x] PR's name and description are correct and up-to-date
  • [x] All temporary labels are removed

[evdokimovs]

FCM

Implement RTC stats for native platforms (#173)

Additionally:
- fix `medea-client-api-proto` fields deserialization to `camelCase`
- fix broken `RtcStats` in Safari