Support for inbox-level settings and Aisauce

[pSpitzner]

• Adds aisauce plugin as a default dependency.
• Adds basic support to configure settings on a per-inbox level. This has great potential to wreak havoc, causing inconsistent libraries if the wrong settings are altered. The focus are plugins. To avoid user errors, config parts that can be altered have to be manually coded into the schema.

[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.

[pSpitzner]

Locally ai_source and ai_cleanup seem to work for me, configured on separate inboxes.

Next steps:

• figure out how to grab logs or states of beets plugins
• test for that (and how to mock ai response?)
• communicate plugin logs and logs to frontend (and well, the beets-flask log, to begin with)