[2.x] Bring media assets into the Hyde kernel

[caendesilva]

This is part of #1904

See if we can cache the media assets during discovery. A good location may be in the kernel Filesystem instance. This would also provide a small performance optimization if we create cache busting keys at discovery time. It would also improve DX with almost no performance penalty as we could throw an exception when trying to get a non-existent file.

Quick benchmarking shows this now goes from ~2.5ms per MediaFile::files() call to ~2.7ms for the first call, and essentially zero at all subsequent calls in the request lifecycle. This is at the cost of the kernel needing to rerun discovery if media files change during the request, which should only happen in unit testing that are coupled to the filesystem.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (314c57a) to head (94e3f21).

Additional details and impacted files

@@                     Coverage Diff                     @@
##             normalize-the-asset-api     #1917   +/-   ##
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  Coverage                     100.00%   100.00%           
- Complexity                      1873      1875    +2     
===========================================================
  Files                            192       193    +1     
  Lines                           4944      4948    +4     
===========================================================
+ Hits                            4944      4948    +4     

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[caendesilva]

Okay, added a more robust benchmarker in f6b8382, with about 15 files, including nested ones, and a 10MB one.

# Before:
Warmup: 15.8036ms
Benchmark: 10.2105987ms # avg/1000/its

# After:
Warmup: 16.9699ms
Benchmark: 0.00093500000000001ms # avg/1000/its