feat: search driver backend extension API

Part of #3884 (https://github.com/flarum/framework/issues/3884#issuecomment-1747042916)

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.

flarum / framework

feat: search driver backend extension API #3902