mpegtsmux: Support metadata in PES packets streams in TS

Fixes #3485 (closed)

Adds support for PES metadata streams in Transport Stream mpegtsmux element.

• Added meta/x-id3, parsed=true caps template to mpegtsmux to mux ID3 metadata buffers into the TS stream.
• Program and elementary stream descriptors are added to the Program Specific Information according to Apple metadata in HLS spec for ID3.
  • A metadata_pointer_descriptor at program level
  • A metadata_descriptor with metadata_format_identifier = 0x49443320 ("ID3 ") metadata_application_format_identifier = 0x49443320 in the Elementary Stream
  • A registration_descriptor with format_identifier = 0x49443320 ("ID3 ").

Running a gst-launch pipeline like

videotestsrc is-live=true ! tee name=video_raw_tee
        metadatasrc is-live=true ! meta/x-id3, parsed=true ! tee name=metadatatee ! queue ! fakesink
        video_raw_tee. ! queue name=video_play_raw_queue ! videoconvert ! autovideosink
        video_raw_tee. ! queue ! videoconvert ! x264enc bitrate=2000 byte-stream=true key-int-max=25 bframes=0 aud=true tune=zerolatency ! video/x-h264,profile=baseline,stream-format=byte-stream ! tee name="video_transcoded_enc_tee"
        video_transcoded_enc_tee. ! queue name="video_transcoded_queue_mpegts" ! mpegtsmux name="mpegts_mux" ! tee name="mpegts_mux_transcoded_tee"
        metadatatee. ! queue ! mpegts_mux.
        mpegts_mux_transcoded_tee. ! queue ! filesink location=out.ts

where the metadatasrc produces text buffers live (25Hz), creates a Transport Stream file out.ts that looks like:

PSI/PMT

PMT descriptors

• (1) metadata_pointer_descriptor in program_info
• (2) Elementary Stream as Metadata carried in PES packets.
• (3) metadata_descriptor attached to Elementary Stream, with metadata_application_format_identifier and metadata_format_identifier set to 0x49443320 (ID3 )
• (4) registration_descriptor attached to Elementary Stream, with format_identifier set to 0x49443320 (ID3 )