iOS: Add numTemporalLayers to RtpEncodingParameters.

Bug: webrtc:9785
Change-Id: I0e57529e8b9aa39d53f27b9b7d6f1d62155d9c34
Reviewed-on: https://webrtc-review.googlesource.com/c/102261
Reviewed-by: Anders Carlsson <andersc@webrtc.org>
Commit-Queue: Rasmus Brandt <brandtr@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24949}

sdk/objc/api/peerconnection/RTCRtpEncodingParameters.h

@@ -35,6 +35,11 @@ RTC_OBJC_EXPORT
  */
 @property(nonatomic, copy, nullable) NSNumber *maxFramerate;
 
+/** The requested number of temporal layers to use for the encoding, or nil
+ * if the default should be used.
+ */
+@property(nonatomic, copy, nullable) NSNumber *numTemporalLayers;
+
 /** The SSRC being used by this encoding. */
 @property(nonatomic, readonly, nullable) NSNumber *ssrc;