[4.x] Add current url resolver to sites

[ajnsn]

When using Livewire components within multisite I noticed that it's not to easy to rely on the Site::current() because the current() relies on the request()->getUri(). In Livewire's subsequent requests this is not the original url which results typically in resolving the default site.

This PR adds the possbility to provide a callback to allow custom url resolution for current().

This could be used in a SeviceProvider using Livewire 3 likes this:

use Statamic\Facades\Site;

Site::resolveCurrentUrl(fn () => Livewire::originalUrl());

[jasonvarga]

Cool idea.

Could you make it non-static?

If you do Statamic\Facades\Site::resolveCurrentUrl() it'll call the resolveCurrentUrl in this class in a non-static way.

The Site (singular) facade calls the Sites (plural) class.

[jasonvarga]

Thanks!

I've renamed it to resolveCurrentUrlUsing to better fit with some Laravel conventions.

e.g. https://github.com/laravel/framework/blob/88d6037dcda4f5696b85ce24ad11ff7e97fd14f0/src/Illuminate/Auth/AuthManager.php#L261-L266