replaygain: add metaflac backend

[SamadBallaj1]

The replaygain plugin can't compute ReplayGain when metaflac is the only tool available. The existing backends (command, gstreamer, audiotools, ffmpeg) each need something heavier installed, which doesn't work on minimal setups like a NAS where those won't install. Issue #1203 asks for a metaflac based option so FLAC users on those setups can still tag ReplayGain.

This adds a metaflac backend for that case. It runs metaflac --add-replay-gain to compute the gain and reads the values back with metaflac --show-tag. I modeled it on the existing CommandBackend since both wrap an external tool. It only handles FLAC, skips other formats, and shifts the gain to the configured targetlevel like the other backends.

One limitation: metaflac scans a whole album in one pass, so the files of an album need the same sample rate and channel layout.

Before, selecting the metaflac backend failed:

$ beet replaygain
UserError: Selected ReplayGain backend metaflac is not supported.

After, the backend works and the plugin tests pass:

$ python -m pytest test/plugins/test_replaygain.py
7 passed, 4 skipped

The 4 skipped are the Opus R128 cases, which don't apply to the metaflac path.

To Do

• Documentation.
• Changelog.
• Tests.

[codecov]

Codecov Report

❌ Patch coverage is 35.08772% with 37 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.67%. Comparing base (08081f3) to head (345b72f).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
beetsplug/replaygain.py	35.08%	36 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6800      +/-   ##
==========================================
- Coverage   74.97%   74.67%   -0.30%     
==========================================
  Files         163      163              
  Lines       20957    21014      +57     
  Branches     3302     3308       +6     
==========================================
- Hits        15712    15693      -19     
- Misses       4485     4563      +78     
+ Partials      760      758       -2     

Files with missing lines	Coverage Δ
beetsplug/replaygain.py	27.01% <35.08%> (+0.68%)	⬆️

... and 1 file with indirect coverage changes

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[SamadBallaj1]

Pushed some updates, this is ready for review whenever you have a chance. Thanks.