Added support for optional extra beets plugins

[FlorentLM]

I'm using https://github.com/BinaryBrain/Beetstream to serve my beets database to Subsonic clients and wanted to have a clean way to install it.

So here are two new environment variables:

PIP_EXTRA_PACKAGES : comma separated list of beets plugins to install via pip
EXTRA_COMMANDS : comma separated list of extra commands to run at the start

Example given in the README:

(...)
environment:
    - UID=1000 # (optionally) set user id
    - GID=1000 # (optionally) set group id
    - PIP_EXTRA_PACKAGES=beetstream
    - EXTRA_COMMANDS="beet beetstream &"
( ...)

The pip install runs via root (because the plugins need to be installed in /usr/lib/python3.11/site-packages otherwise beets won't see them, but the root account is immediately dropped after that and then the extra commands and the main betanin command are ran under the unproviledged user.

Edit: Oh, and I removed the dependency to sudo because it's all running as the betanin user