[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/Headers/WebRTC/RTCConfiguration.h

@@ -109,7 +109,7 @@ RTC_EXPORT
  */
 @property(nonatomic, copy, nullable) NSNumber *iceCheckMinInterval;
 
-- (instancetype)init NS_DESIGNATED_INITIALIZER;
+- (instancetype)init;
 
 @end