Make Coturn optional (#182)

[evdokimovs]

Part of #182

Synopsis

We need to make Coturn dependency optional. So after this PR Medea can work without Coturn and Redis.

Solution

Add ice.default and [ice.static] to the configuration options. When ice.default is "static", then Coturn wouldn't be used at all. Only credentials provided in [ice.static] will be provided to the clients.

Checklist

• Created PR:
  • In draft mode
  • Name contains Draft: 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
  • Draft: prefix is removed
  • All temporary labels are removed

[evdokimovs]

FCM

Make Coturn optional (#211, #182)

- add static STUN/TURN credentials support
- rename [turn] config section as [ice]

Additionally:
- change Flutter channel to 'beta' on CI
- remove deprecated 'author' field from the Flutter's spec

[evdokimovs]

Убрал author поле из Flutter pubspec.yml, потому-что на него стал ругаться dartanalyzer из-за того, что его задепрекейтили недавно.

dart-lang/site-www#2029