Add an example app for iOS native API.

Demonstrates how to use the iOS native API to wrap components into
C++ classes.

This CL also introduces a native API wrapper for the capturer.

The C++ code is forked from the corresponding CL for Android at
https://webrtc-review.googlesource.com/c/src/+/60540

Bug: webrtc:8832
Change-Id: I12d9f30e701c0222628e329218f6d5bfca26e6e0
Reviewed-on: https://webrtc-review.googlesource.com/61422
Commit-Queue: Anders Carlsson <andersc@webrtc.org>
Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22484}

sdk/objc/Framework/Headers/WebRTC/RTCVideoCapturer.h

@@ -22,7 +22,7 @@ RTC_EXPORT
 RTC_EXPORT
 @interface RTCVideoCapturer : NSObject
 
-@property(nonatomic, readonly, weak) id<RTCVideoCapturerDelegate> delegate;
+@property(nonatomic, weak) id<RTCVideoCapturerDelegate> delegate;
 
 - (instancetype)initWithDelegate:(id<RTCVideoCapturerDelegate>)delegate;