Implement video codec HW acceleration probing on iOS

[evdokimovs]

Synopsis

We want to use HW acceleration for the video when it possible. For this purpose was introduced PeerConnectionFactory.videoEncoders and PeerConnectionFactory.videoDecoders APIs which provides information about available codecs and their HW acceleration capability. But at this moment this API is not implemented on iOS, so this PR implements them.

Firstly, this PR was conceived mainly as HW acceleration implementation of VP9 codec for iOS devices, but it was discovered that VP8/VP9 codecs are not officially supported on this devices. There is undocumented feature mostly exclusively added for the YouTube iOS app which adds support for VP9 codec HW decoding, but it looks not very promising for us, because there is no evidence that someone used it elsewhere and it can lead to the problems while publishing to App Store. But here is repo with some more info about it: https://github.com/PoomSmart/YTUHD

Solution

Implement PeerConnectionFactory.videoEncoders and PeerConnectionFactory.videoDecoders API on iOS side using VTIsHardwareDecodeSupported.

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
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  • All temporary labels are removed

[evdokimovs]

FCM

Implement video codec HW acceleration probing on iOS (#148)