Plugin & Shell: improve filter_state initialization

Context

Since the shell uses the new protocol mempool (!6749 (merged)), we need to call both Proto.Mempool.init and Proto.begin_validation in prevalidation.ml, even though both functions do almost the same thing. This is because several functions in the mempool plugin expect a validation_state, which otherwise isn't useful anymore in the new mempool.

This MR reworks the mempool plugin functions so that they don't require a validation_state anymore, allowing us to finally remove the Proto.begin_validation from prevalidation.ml.

This MR also does some preliminary refactoring in prevalidator_internal.ml and prevalidation.ml to reduce the scope of the interface with the plugin and make the aforementioned changes easier.

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• [N/A] Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• [N/A] For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR