feat: conditional extenders

[SychO9]

Changes proposed in this pull request:
Adds support for conditional extenders, needed especially when adding extenders when certain extensions are enabled. (needed for tag mentions).

Here are some usage examples:

return [
    (new Extend\Conditional)
        ->whenExtensionEnabled('flarum-tags', [
                (new Extend\Formatter)
                    ->parse(FormatTagMentions::class)
                    ->unparse(UnparseTagMentions::class),

                (new Extend\ApiController(ListPostsController::class)
                    ->load('mentionsTags'),

                (new Extend\Model(Post::class)
                    ->belongsToMany('mentionsTags', Tag::class, 'post_mentions_tag', 'post_id', 'mentions_tag_id'),

                (new Extend\ApiSerializer(BasicPostSerializer::class))
                    ->hasMany('mentionsTags', TagSerializer::class),
        ]),
]

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.

Required changes:

• Related documentation PR: (Remove if irrelevant)
• Related core extension PRs: (Remove if irrelevant)

[imorland]

I've been considering this for a while now actually. Great job 👍

[SychO9]

Inspired by @davwheat & @luceos 🤗

[luceos]

Good job. Please do cleanups in different pr's next time, makes reviewing a ton easier 🙈

[luceos]

The signature doesn't match the PR comment example. Your example uses a callable as the second argument, not an array. The array is the return response from the callable yes. I assume your example is wrong then..

Also the phpdoc is incorrect, there are two args.

[SychO9]

yea updated the example thanks

phpdoc doesn't matter, we only add typings for what isn't typed, we can't vanilla type array as ExtenderInterface[], we've being doing this since 1.0 to avoid unnecessary duplication. And the phpdoc standard allows it.