Implement read-only codecPayloadType in RtpParameters

Bug: webrtc:7580
Change-Id: I6d901afa97262b6c6d9fe6c7366df465ec77bfb3
Reviewed-on: https://webrtc-review.googlesource.com/c/113944
Reviewed-by: Sami Kalliomäki <sakal@webrtc.org>
Reviewed-by: Seth Hampson <shampson@webrtc.org>
Reviewed-by: Anders Carlsson <andersc@webrtc.org>
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Commit-Queue: Florent Castelli <orphis@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25993}

sdk/objc/api/peerconnection/RTCRtpEncodingParameters.h

@@ -17,6 +17,11 @@ NS_ASSUME_NONNULL_BEGIN
 RTC_OBJC_EXPORT
 @interface RTCRtpEncodingParameters : NSObject
 
+/** The codec payloadType used by the encoder, or nil if it is not currently
+ * available.
+ */
+@property(nonatomic, readonly, nullable) NSNumber *codecPayloadType;
+
 /** Controls whether the encoding is currently transmitted. */
 @property(nonatomic, assign) BOOL isActive;