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

@@ -330,6 +330,7 @@ dictionary RTCEncodedVideoFrameMetadata {
     long temporalIndex;
     unsigned long synchronizationSource;
     octet payloadType;
+    DOMString mimeType;
     sequence<unsigned long> contributingSources;
 };
 </pre>
@@ -433,6 +434,7 @@ interface RTCEncodedVideoFrame {
 dictionary RTCEncodedAudioFrameMetadata {
     unsigned long synchronizationSource;
     octet payloadType;
+    DOMString mimeType;
     sequence&lt;unsigned long&gt; contributingSources;
 };
 </pre>