Pydantic models for Plugin Config

[SpecialBuilder32]

This PR refactors how our beet.yaml metadata is read into many of our build plugins, using the intended system of pydantic models rather than arbitrary dict access. This change allows for a much clearer definition of what options are available/expected and in what format.

Also included in this PR (and its impetus), is the addition of the search_keywords config option for the website.

[RitikShah]

I skimmed through it, happy to review when you think it's ready. I would stick with either Optional[type] or type | None (whichever you prefer),

BTW, beet is still on Pydantic v1.x and not v2.x so make sure you are reading the correct docs for that!

[SpecialBuilder32]

Yep! I leaned pydantic while working on the RP merge, which has a by far more complex set of models.

I've been using str|None when the expected value is either null or foo, and Optional[] for when the argument isn't required to be set

..and yes I fear the update to pydantic 2 when I'll have to redo everything :(

[Bloo-dev]

I can't find any reference to search_keywords in the code of this PR, did you forget to commit that?

[SpecialBuilder32]

I can't find any reference to search_keywords in the code of this PR, did you forget to commit that?

I'm still working on converting to the new models and verifying that doesn't break anything before adding new stuff