clipping: search for first and last segments to clip

[emranemran]

Part 1 in series of Clipping related PRs.

This change adds methods to take any HLS manifest and return the start and end segments that correspond to the specified clipping timestamps.

[codecov]

Codecov Report

Merging #777 (30bf07d) into main (5662003) will increase coverage by 0.10839%.
The diff coverage is 67.85714%.

@@                 Coverage Diff                 @@
##                main        #777         +/-   ##
===================================================
+ Coverage   58.19323%   58.30162%   +0.10839%     
===================================================
  Files             58          59          +1     
  Lines           4937        4993         +56     
===================================================
+ Hits            2873        2911         +38     
- Misses          1843        1855         +12     
- Partials         221         227          +6     

Files Changed	Coverage Δ
video/clip.go	67.85714% <67.85714%> (ø)

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Files Changed	Coverage Δ
video/clip.go	67.85714% <67.85714%> (ø)

[emranemran]

Updates:

• ClipManifest now returns a slice of m3u8.MediaSegment that contains all segments from the specified start time to end time
• getRelevantSegment now returns just indices and doesn't use any pointers (even though my brain sometimes prefers passing around pointers).
• Added back getTotalDurationAndSegments function that's useful to simplify the range loop
• Updated tests to match the new logic

Pls re-review @thomshutt @mjh1