Refactor to use a Provider abstraction

[jeffcharles]

Description of the change

I'm adding a Provider abstraction which provides methods that need to be polymorphic over a provider.

Why am I making this change?

As part of #768 I'll need to thread a plugin module through a bunch of the code. This refactoring front-loads some of those changes.

Checklist

• I've updated the relevant CHANGELOG files if necessary. Changes to javy-cli and javy-core do not require updating CHANGELOG files.
• I've updated the relevant crate versions if necessary. Versioning policy for library crates
• I've updated documentation including crate documentation if necessary.

[saulecabrera]

Looks reasonable to me!