Change RTPVideoTypeHeader to absl::variant and move RTPVideoHeader into its own h/cc file.

Bug: none
Change-Id: If28f57c5ae250afbb47c5d20c9854e9a11182642
Reviewed-on: https://webrtc-review.googlesource.com/87561
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Philip Eliasson <philipel@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23904}

modules/video_coding/packet.cc

@@ -86,7 +86,7 @@ void VCMPacket::Reset() {
   insertStartCode = false;
   width = 0;
   height = 0;
-  memset(&video_header, 0, sizeof(RTPVideoHeader));
+  video_header = {};
 }
 
 void VCMPacket::CopyCodecSpecifics(const RTPVideoHeader& videoHeader) {