add mimeType to metadata

since the mapping from payload type to the codec mime type
is nontrivial in environments like workers.

Defined similar to webrtc-stats:
  https://w3c.github.io/webrtc-stats/#dom-rtccodecstats-mimetype

Author: fippo

index.bs

@@ -294,6 +294,7 @@ dictionary RTCEncodedVideoFrameMetadata {
     long temporalIndex;
     long synchronizationSource;
     octet payloadType;
+    DOMString mimeType;
     sequence<long> contributingSources;
 };
 
@@ -310,6 +311,7 @@ interface RTCEncodedVideoFrame {
 dictionary RTCEncodedAudioFrameMetadata {
     long synchronizationSource;
     octet payloadType;
+    DOMString mimeType;
     sequence<long> contributingSources;
 };