Eager loading extender

[SychO9]

Part of #2637

Changes proposed in this pull request:
Adds a load() method on the ApiController extender that allows extensions to specify relations that need to be loaded in listing controllers. This works when controllers use the appropriate loadRelations($collections, $relations) method.

A first level relationship will always be loaded, whereas a sublevel relationship will only be loaded if the parent level is/will be loaded.

As for the noticiations endpoint that has the subject polymorphic relationship, I would like to add in another pull request a loadMorph method to this same extender that would solve the problem.

Reviewers should focus on:

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).
• Related core extension PRs: (Remove if irrelevant).
  • Eager load tags relation in discussion, posts and flags listing endpoints tags#125
  • Eager load mentionedBy and only missing relations mentions#64
  • Eager load post.user.groups relation and allow extensions to eager load relations flags#38

[askvortsov1]

This will need a bit in the extension development docs, so that we can point extension devs to an explanation of how to use it.