ai/live: Add segmented MediaWriter / MediaReader

[j0sh]

This allows us to stream segments from one in-memory location to another. New reads will always start at the beginning of the current segment.

Also make the original MediaWriter follow io.WriteCloser

Needed for #3689

[codecov]

Codecov Report

❌ Patch coverage is 95.06173% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 31.70296%. Comparing base (4044283) to head (cc18b19).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
media/segment_rw.go	96.15385%	2 Missing and 1 partial ⚠️
media/rw.go	66.66667%	1 Missing ⚠️

Additional details and impacted files

@@                 Coverage Diff                 @@
##              master       #3705         +/-   ##
===================================================
+ Coverage   31.59467%   31.70296%   +0.10829%     
===================================================
  Files            157         158          +1     
  Lines          47527       47617         +90     
===================================================
+ Hits           15016       15096         +80     
- Misses         31617       31626          +9     
- Partials         894         895          +1     

Files with missing lines	Coverage Δ
media/rw.go	90.14085% <66.66667%> (+0.14085%)	⬆️
media/segment_rw.go	96.15385% <96.15385%> (ø)

... and 4 files with indirect coverage changes

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Files with missing lines	Coverage Δ
media/rw.go	90.14085% <66.66667%> (+0.14085%)	⬆️
media/segment_rw.go	96.15385% <96.15385%> (ø)

... and 4 files with indirect coverage changes

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