Add RegisterExternalDecoder in VideoCodingModule.

In preparation for landing https://webrtc-review.googlesource.com/c/src/+/72441
a downstream project that uses the VideoCodingModule needs to be able to
inject a decoder object created from the outside, just like how encoders
are possible to inject.

Bug: webrtc:7925
Change-Id: Ibaeffda55f84410436d79f75730e7352e298b9f0
Reviewed-on: https://webrtc-review.googlesource.com/77160
Commit-Queue: Anders Carlsson <andersc@webrtc.org>
Reviewed-by: Rasmus Brandt <brandtr@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23297}

modules/video_coding/video_coding_impl.cc

@@ -163,6 +163,11 @@ class VideoCodingModuleImpl : public VideoCodingModule {
                                           requireKeyFrame);
   }
 
+  void RegisterExternalDecoder(VideoDecoder* externalDecoder,
+                               uint8_t payloadType) override {
+    receiver_.RegisterExternalDecoder(externalDecoder, payloadType);
+  }
+
   int32_t RegisterReceiveCallback(
       VCMReceiveCallback* receiveCallback) override {
     RTC_DCHECK(construction_thread_.CalledOnValidThread());