Adding getParameters/setParameters APIs to RtpReceiver.

This is similar to how a "receive" method is used to apply
RtpParameters to an RtpReceiver in ORTC. Currently, SetParameters
doesn't allow changing the parameters, so the main use of the API is
to retrieve the set of configured codecs. But other uses will likely
be made possible in the future.

R=glaznev@webrtc.org, pthatcher@webrtc.org, tkchin@webrtc.org

Review URL: https://codereview.webrtc.org/1917193008 .

Cr-Commit-Position: refs/heads/master@{#12761}

webrtc/sdk/objc/Framework/Classes/RTCPeerConnection.mm

@@ -17,6 +17,7 @@
 #import "RTCMediaConstraints+Private.h"
 #import "RTCMediaStream+Private.h"
 #import "RTCPeerConnectionFactory+Private.h"
+#import "RTCRtpReceiver+Private.h"
 #import "RTCRtpSender+Private.h"
 #import "RTCSessionDescription+Private.h"
 #import "RTCStatsReport+Private.h"
@@ -342,6 +343,18 @@ void PeerConnectionDelegateAdapter::OnIceCandidate(
   return senders;
 }
 
+- (NSArray<RTCRtpReceiver *> *)receivers {
+  std::vector<rtc::scoped_refptr<webrtc::RtpReceiverInterface>> nativeReceivers(
+      _peerConnection->GetReceivers());
+  NSMutableArray *receivers = [[NSMutableArray alloc] init];
+  for (const auto &nativeReceiver : nativeReceivers) {
+    RTCRtpReceiver *receiver =
+        [[RTCRtpReceiver alloc] initWithNativeRtpReceiver:nativeReceiver];
+    [receivers addObject:receiver];
+  }
+  return receivers;
+}
+
 #pragma mark - Private
 
 + (webrtc::PeerConnectionInterface::SignalingState)nativeSignalingStateForState: