Wrap WebRTC OBJC API types with RTC_OBJC_TYPE.

This CL introduced 2 new macros that affect the WebRTC OBJC API symbols:

- RTC_OBJC_TYPE_PREFIX:
  Macro used to prepend a prefix to the API types that are exported with
  RTC_OBJC_EXPORT.

  Clients can patch the definition of this macro locally and build
  WebRTC.framework with their own prefix in case symbol clashing is a
  problem.

  This macro must only be defined by changing the value in
  sdk/objc/base/RTCMacros.h  and not on via compiler flag to ensure
  it has a unique value.

- RCT_OBJC_TYPE:
  Macro used internally to reference API types. Declaring an API type
  without using this macro will not include the declared type in the
  set of types that will be affected by the configurable
  RTC_OBJC_TYPE_PREFIX.

Manual changes:
https://webrtc-review.googlesource.com/c/src/+/173781/5..10

The auto-generated changes in PS#5 have been done with:
https://webrtc-review.googlesource.com/c/src/+/174061.

Bug: None
Change-Id: I0d54ca94db764fb3b6cb4365873f79e14cd879b8
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/173781
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31153}

sdk/objc/api/video_codec/RTCVideoEncoderVP9.h

@@ -14,12 +14,12 @@
 #import "RTCVideoEncoder.h"
 
 RTC_OBJC_EXPORT
-@interface RTCVideoEncoderVP9 : NSObject
+@interface RTC_OBJC_TYPE (RTCVideoEncoderVP9) : NSObject
 
 /* This returns a VP9 encoder that can be returned from a RTCVideoEncoderFactory injected into
  * RTCPeerConnectionFactory. Even though it implements the RTCVideoEncoder protocol, it can not be
  * used independently from the RTCPeerConnectionFactory.
  */
-+ (id<RTCVideoEncoder>)vp9Encoder;
++ (id<RTC_OBJC_TYPE(RTCVideoEncoder)>)vp9Encoder;
 
 @end