Port transcode and musicbrainz plugins into this repository

[jtpavlock]

This moves both the transcode and musicbrainz plugins, which were previously separate repositories, into moe as "official" plugins. Official plugins are separated with moe from core repositories by living in the "plugins" directory.

The following breaking changes arise from this:

1. Importing moe_transcode and moe_musicbrainz

   Imports for these modules have changed to import moe.plugins.transcode and import moe.plugins.musicbrainz.

2. Installation

   Official plugins must now be installed as extras within moe. To install with pipx:
   pipx install moe[musicbrainz]. Because the transcode has no additional (python) dependencies, there's no need to install anything extra with pip.

For more discussion around this change, see #319.

Of note, configuration values do not change i.e. you still use [transcode] or [musicbrainz] blocks to set values all the same.

I'm also open to changing how I've laid out the structure. I resorted to putting official plugins in their own "plugins" directory to physically separate them from the core plugins, but I'm not sure if that's necessary and if it would be better to just have them all in the same directory and import them as moe.musicbrainz instead of moe.plugins.musicbrainz.

For code changes, besides moving the files, the following changes were made:

• Incorporated ruff checks into both plugins
• Added a new pytest marker ffmpeg to mark which tests require this commandline dependency.
• Imports fixed to reflect the new structure
• docs for both plugins now belong in a single official_plugins.rst file
• install docs for both plugins have been updated. because the musicbrainz plugin requires an additional python dependency, musicbrainzngs, it's been marked as an optional extra in poetry.

---

📚 Documentation preview 📚: https://mrmoe--321.org.readthedocs.build/en/321/

[codecov]

Codecov Report

❌ Patch coverage is 97.67442% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.96%. Comparing base (b7d5963) to head (3362e78).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #321      +/-   ##
==========================================
+ Coverage   93.36%   93.96%   +0.59%     
==========================================
  Files          53       58       +5     
  Lines        2443     2785     +342     
==========================================
+ Hits         2281     2617     +336     
- Misses        162      168       +6     

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[ali-ramadhan]

Looks good to me! I mostly left some questions and comments.

[jtpavlock]

Thanks for the review @ali-ramadhan. If you like I can wait until #317 gets merged before merging this pr so we can make a new minor feature release of moe, or I can merge this one now if you're not in a rush. There's a couple other small breaking change PRs I'd like to make before the v3.0 release goes out after this one gets merged in.

[ali-ramadhan]

I'm not in a rush and I don't think there will be any conflicts between the two PRs. Feel free to merge either first!