Add new PeerConnection APIs to the ObjC SDK

This CL adds wrappers for the following PeerConnection native
APIs to the Objective C API:
- SdpSemantics enum added to the RTCConfiguration
- RTCRtpTransceiver
- RTCPeerConnection.addTrack
- RTCPeerConnection.removeTrack
- RTCPeerConnection.addTransceiver
- RTCPeerConnection.transceivers

Bug: webrtc:8870
Change-Id: I9449df9742a59e90894712dc7749ca30b569d94b
Reviewed-on: https://webrtc-review.googlesource.com/54780
Commit-Queue: Steve Anton <steveanton@webrtc.org>
Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22214}

sdk/objc/Framework/Headers/WebRTC/RTCConfiguration.h

@@ -63,6 +63,13 @@ typedef NS_ENUM(NSInteger, RTCEncryptionKeyType) {
   RTCEncryptionKeyTypeECDSA,
 };
 
+/** Represents the chosen SDP semantics for the RTCPeerConnection. */
+typedef NS_ENUM(NSInteger, RTCSdpSemantics) {
+  RTCSdpSemanticsDefault,
+  RTCSdpSemanticsPlanB,
+  RTCSdpSemanticsUnifiedPlan,
+};
+
 NS_ASSUME_NONNULL_BEGIN
 
 RTC_EXPORT
@@ -132,6 +139,33 @@ RTC_EXPORT
  */
 @property(nonatomic, strong, nullable) RTCIntervalRange *iceRegatherIntervalRange;
 
+/** Configure the SDP semantics used by this PeerConnection. Note that the
+ *  WebRTC 1.0 specification requires UnifiedPlan semantics. The
+ *  RTCRtpTransceiver API is only available with UnifiedPlan semantics.
+ *
+ *  PlanB will cause RTCPeerConnection to create offers and answers with at
+ *  most one audio and one video m= section with multiple RTCRtpSenders and
+ *  RTCRtpReceivers specified as multiple a=ssrc lines within the section. This
+ *  will also cause RTCPeerConnection to ignore all but the first m= section of
+ *  the same media type.
+ *
+ *  UnifiedPlan will cause RTCPeerConnection to create offers and answers with
+ *  multiple m= sections where each m= section maps to one RTCRtpSender and one
+ *  RTCRtpReceiver (an RTCRtpTransceiver), either both audio or both video. This
+ *  will also cause RTCPeerConnection to ignore all but the first a=ssrc lines
+ *  that form a Plan B stream.
+ *
+ *  For users who only send at most one audio and one video track, this
+ *  choice does not matter and should be left as Default.
+ *
+ *  For users who wish to send multiple audio/video streams and need to stay
+ *  interoperable with legacy WebRTC implementations, specify PlanB.
+ *
+ *  For users who wish to send multiple audio/video streams and/or wish to
+ *  use the new RTCRtpTransceiver API, specify UnifiedPlan.
+ */
+@property(nonatomic, assign) RTCSdpSemantics sdpSemantics;
+
 - (instancetype)init;
 
 @end