[iOS] Added an initialization method to RTCConfiguration that takes a

native configuration.
Added a getConfiguration getter method to RTCPeerConnection to return
the RTCConfiguration.

BUG=webrtc:7431

Review-Url: https://codereview.webrtc.org/2790833002
Cr-Commit-Position: refs/heads/master@{#17517}

webrtc/sdk/objc/Framework/Classes/RTCConfiguration+Private.h

@@ -65,6 +65,9 @@ NS_ASSUME_NONNULL_BEGIN
 - (webrtc::PeerConnectionInterface::RTCConfiguration *)
     createNativeConfiguration;
 
+- (instancetype)initWithNativeConfiguration:
+    (const webrtc::PeerConnectionInterface::RTCConfiguration *)config NS_DESIGNATED_INITIALIZER;
+
 @end
 
 NS_ASSUME_NONNULL_END