Add bindings for RIDs in iOS SDK.

This adds bindings for RIDs in RtpEncodingParameters.

Bug: webrtc:10464
Change-Id: I3cc25db25a4d777b9d9573ba69c82127d1c9a597
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/128826
Reviewed-by: Anders Carlsson <andersc@webrtc.org>
Commit-Queue: Amit Hilbuch <amithi@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#27322}

sdk/objc/api/peerconnection/RTCRtpEncodingParameters.h

@@ -17,6 +17,9 @@ NS_ASSUME_NONNULL_BEGIN
 RTC_OBJC_EXPORT
 @interface RTCRtpEncodingParameters : NSObject
 
+/** The idenfifier for the encoding layer. This is used in simulcast. */
+@property(nonatomic, copy, nullable) NSString *rid;
+
 /** Controls whether the encoding is currently transmitted. */
 @property(nonatomic, assign) BOOL isActive;