feat: search driver backend extension API

[SychO9]

Part of #3884 (#3884 (comment))

Changes proposed in this pull request:
Introduces a search driver API. (Checkout the POC here: https://github.com/SychO9/flarum-ext-search/blob/main/extend.php#L40-L48).

    (new Flarum\SearchDriver(Search\ElasticSearchDriver::class))
        ->addSearcher(FlarumDiscussion::class, Discussion\DiscussionSearcher::class)
        ->addFilter(Discussion\DiscussionSearcher::class, Discussion\PrivateFilterMutator::class)
        ->addMutator(Discussion\DiscussionSearcher::class, Discussion\PrivateFilterMutator::mutate(...))
        ->setFulltext(Discussion\DiscussionSearcher::class, Discussion\FulltextFilter::class),

    (new Flarum\SearchIndex())
        ->indexer(FlarumDiscussion::class, Discussion\DiscussionIndexer::class)
        ->indexer(FlarumPost::class, Post\CommentPostIndexer::class),

• Search drivers declare their own DriverInterface.
• Search drivers declare their own Searcher per model.
• Search drivers can optionally declare a model indexer (IndexerInterface).
• Search drivers may use their own SearchState implementation. (use of phpstan template for generics helps with typings here).

Follow up PRs
The follow up PR will include

• Adding an advanced admin page where drivers can be configured per model.

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.

[SychO9]

#3893