Enable padding bit in TransportFeedback packets

Set padding bit if the TransportFeedback packet contains zero padding.
Also write number of padding elements at the last position of the packet.

Bug: webrtc:10263
Change-Id: I8d17bc0e889f658ac383dec64ddcb95d438c9702
Reviewed-on: https://webrtc-review.googlesource.com/c/122240
Reviewed-by: Erik Språng <sprang@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Johannes Kron <kron@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26646}

modules/rtp_rtcp/source/rtcp_packet.h

@@ -87,6 +87,13 @@ class RtcpPacket {
                            uint8_t* buffer,
                            size_t* pos);
 
+  static void CreateHeader(size_t count_or_format,
+                           uint8_t packet_type,
+                           size_t block_length,  // Payload size in 32bit words.
+                           bool padding,  // True if there are padding bytes.
+                           uint8_t* buffer,
+                           size_t* pos);
+
   bool OnBufferFull(uint8_t* packet,
                     size_t* index,
                     PacketReadyCallback callback) const;