produce same absolute time in RTSP and HLS

[aler9]

• add a NTP timestamp field to each data unit
• use that NTP timestamp in all protocols

[codecov-commenter]

Codecov Report

Merging #1249 (b5b6868) into main (282d155) will decrease coverage by 0.06%.
The diff coverage is 72.22%.

@@            Coverage Diff             @@
##             main    #1249      +/-   ##
==========================================
- Coverage   67.51%   67.44%   -0.07%     
==========================================
  Files         107      107              
  Lines        8917     8939      +22     
==========================================
+ Hits         6020     6029       +9     
- Misses       2327     2340      +13     
  Partials      570      570              

Impacted Files	Coverage Δ
internal/core/hls_muxer.go	58.88% <0.00%> (ø)
internal/core/rpicamera_source.go	0.00% <0.00%> (ø)
internal/core/data.go	33.33% <33.33%> (ø)
internal/core/rtmp_conn.go	64.96% <33.33%> (-0.24%)	⬇️
internal/core/rtsp_session.go	64.95% <33.33%> (-0.42%)	⬇️
internal/core/rtsp_source.go	70.37% <33.33%> (-1.06%)	⬇️
internal/core/hls_source.go	71.42% <50.00%> (-0.64%)	⬇️
internal/core/rtmp_source.go	71.30% <50.00%> (-0.38%)	⬇️
internal/core/stream.go	92.85% <100.00%> (ø)
internal/hls/muxer.go	76.74% <100.00%> (ø)
... and 7 more

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