Add ability to download libwebrc-bin from branch for mobile

[turtletongue]

Part of #224

Synopsis

There is a need to run CI on pre-release branches of the libwebrtc-bin. Artifacts from last libwebrtc-bin CI run should be accessible as easy as release artifacts.

Solution

A release or a branch is selected based on WEBRTC_BRANCH environment variable. Empty WEBRTC_BRANCH is used in release builds and non-empty WEBRTC_BRANCH ensures use of artifacts from last green CI of specific branch. Downloading of artifacts is based on GitHub API, so GH_TOKEN environment variable must also be set if WEBRTC_BRANCH is not empty.

Example of usage for Android:

WEBRTC_BRANCH=136.0.7103.113 GH_TOKEN=put_token_here make flutter.test.mobile platform=android

Example of usage for IOS:

WEBRTC_BRANCH=136.0.7103.113 GH_TOKEN=put_token_here make flutter.test.mobile platform=ios device=...

Checklist

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

[turtletongue]

FCM

Allow downloading `libwebrc-bin` from branch for mobile targets in `libwebrtc-sys` crate (#238, #224)