Added rsgain ReplayGain backend

[teadoku]

Description

Adds new rsgain ReplayGain backend, which utilises the rsgain CLI tool currently used by the MusicBrainz Picard tagging tool for calculating ReplayGain 2.0 values. It supports a wide variety of formats and R128 tagging, works in both sample and true peak mode, can be used in parallel and also allows control of the maximum peak value allowed when using clipping protection. All existing replaygain configuration values are used, with a new max_peak configuration value that controls the clipping protection as mentioned beforehand.

Full list of changes:

• Added RSGainBackend class that implements the Backend abstract class.
• Added the backend class to the supported list of backends for the main ReplayGainPlugin class.
• Added max_peak float configuration value that controls the maximum allowed peak, in decibels, when clipping protection is enabled via the noclip boolean configuration value. Default value is -1.0 as recommended by the EBU R 128 specification.
• Updated test_replaygain.py with tests for new backend functionality.
• Updated replaygain.rst documentation with details about the new backend and configuration value.
• Updated changelog.rst

To Do

• Documentation.
• Changelog.
• Tests. Note: I did not add a test for the new backend using the "full" example file, which is complete silence, as the rsgain tool returns 0.0 for the gain and negative infinity for the peak. This fails the assert max(gains) != 0.0 assertion.

[github-actions]

Thank you for the PR! The changelog has not been updated, so here is a friendly reminder to check if you need to add an entry.

[codecov]

Codecov Report

❌ Patch coverage is 19.29825% with 46 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.99%. Comparing base (31c917b) to head (4046e11).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
beetsplug/replaygain.py	19.29%	45 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6948      +/-   ##
==========================================
- Coverage   76.14%   75.99%   -0.16%     
==========================================
  Files         163      163              
  Lines       21580    21636      +56     
  Branches     3337     3344       +7     
==========================================
+ Hits        16433    16442       +9     
- Misses       4347     4392      +45     
- Partials      800      802       +2     

Files with missing lines	Coverage Δ
beetsplug/replaygain.py	45.01% <19.29%> (-1.93%)	⬇️

... and 1 file with indirect coverage changes

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