Add ObjC interface wrapping new GetImplementations method.

Bug: webrtc:10795
Change-Id: I32a4bcb9bd51155b6bc82a161765b5cda9539100
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/150100
Commit-Queue: Kári Helgason <kthelgason@webrtc.org>
Reviewed-by: Anders Carlsson <andersc@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28947}
diff --git a/sdk/objc/base/RTCVideoEncoderFactory.h b/sdk/objc/base/RTCVideoEncoderFactory.h
index 20c603d..3291797 100644
--- a/sdk/objc/base/RTCVideoEncoderFactory.h
+++ b/sdk/objc/base/RTCVideoEncoderFactory.h
@@ -23,6 +23,9 @@
 - (nullable id<RTCVideoEncoder>)createEncoder:(RTCVideoCodecInfo *)info;
 - (NSArray<RTCVideoCodecInfo *> *)supportedCodecs;  // TODO(andersc): "supportedFormats" instead?
 
+@optional
+- (NSArray<RTCVideoCodecInfo *> *)implementations;
+
 @end
 
 NS_ASSUME_NONNULL_END