feat: support custom reusable setting components for buildSettingComponent

[davwheat]

Changes proposed in this pull request:

Adds the ability for extensions to declare custom setting components for use with buildSettingComponent.

This could be used to aid model selection, for example, such as providing an easy and reusable way to allow extensions to interact with each other through their settings pages.

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 core extension PRs: feat: add tag selector custom setting component for buildSettingComponent #3495