Reland "Simple Default ObjC video codec factories."

This is a reland of 30915a742d86df55ac5c04501c0e8104675a612e
Original change's description:
> Simple Default ObjC video codec factories.
> 
> Move the simple video encoder/decoder factory from AppRTCMobile into the
> public API so users who don't have special requirements for video codecs
> can easily get started.
> 
> Also clean up the API a little.
> 
> This CL replaces the more flexible default factories in
> https://webrtc-review.googlesource.com/c/src/+/7741 and clients that
> want to implement their own codecs will have to supply their own
> encoder/decoder factories as well. The benefits of the approach in
> this CL are a simpler API and less effects on the rest of the code.
> 
> Bug: None
> Change-Id: I4ed94090d778b4fc38b49864de1d4de4ff125d6a
> Reviewed-on: https://webrtc-review.googlesource.com/15141
> Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
> Reviewed-by: Magnus Jedvert <magjed@webrtc.org>
> Commit-Queue: Anders Carlsson <andersc@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#20441}

Bug: None
Change-Id: If0910cc540dc835dfec4eeb5bea527d88482d110
Reviewed-on: https://webrtc-review.googlesource.com/16780
Reviewed-by: Magnus Jedvert <magjed@webrtc.org>
Commit-Queue: Anders Carlsson <andersc@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20476}

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

@@ -33,4 +33,22 @@ RTC_EXPORT
 
 @end
 
+#pragma mark - Default factories
+
+/** These codec factories include support for all codecs bundled with WebRTC. If using custom
+ *  codecs, create custom implementations of RTCVideoEncoderFactory and RTCVideoDecoderFactory.
+ */
+RTC_EXPORT
+@interface RTCDefaultVideoEncoderFactory : NSObject <RTCVideoEncoderFactory>
+
+@property(nonatomic, retain) RTCVideoCodecInfo *preferredCodec;
+
++ (NSArray<RTCVideoCodecInfo *> *)supportedCodecs;
+
+@end
+
+RTC_EXPORT
+@interface RTCDefaultVideoDecoderFactory : NSObject <RTCVideoDecoderFactory>
+@end
+
 NS_ASSUME_NONNULL_END