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}
diff --git a/video/BUILD.gn b/video/BUILD.gn
index 4cf52a8..5f5f12d 100644
--- a/video/BUILD.gn
+++ b/video/BUILD.gn
@@ -72,6 +72,7 @@
     "../call:rtp_interfaces",
     "../call:video_stream_api",
     "../modules/rtp_rtcp:rtp_rtcp_format",
+    "../modules/rtp_rtcp:rtp_video_header",
     "../modules/video_coding:codec_globals_headers",
     "../modules/video_coding:nack_module",
     "../modules/video_coding:packet",