[ObjC] Expose properties / methods required for AV1 codec support

[hiroshihorie]

• New codecPreferences on RTCRtpTransceiver.
• New scalabilityMode on RTCRtpEncodingParameters.
• New RTCRtpCodecCapability class, ObjC version of webrtc::RtpCodecCapability.
• New sender / receiver capabilities API on RTCPeerConnectionFactory :

• (NSArray<RTC_OBJC_TYPE(RTCRtpCodecCapability) > )rtpSenderCapabilities:(RTCRtpMediaType)mediaType;
• (NSArray<RTC_OBJC_TYPE(RTCRtpCodecCapability) > )rtpReceiverCapabilities:(RTCRtpMediaType)mediaType;

• Expose kAv1CodecName const.
• Always include dependency descriptor in offer.

TODO

• [x] Expose more properties of RTCRtpCodecCapability.
• [x] Error handling.
• [x] Swap header of new files to LK header.
• [ ] More comments.

[cnderrauber]

Looks good! Dose android need export these too?

[hiroshihorie]

Looks good! Dose android need export these too?

We decided to make another PR for that .