Fix filtering on private api routes

[godismyjudge95]

Currently filtering does not work because none of the controllers implement the required allowedFilters method:
https://github.com/statamic/cms/blob/5.x/src/Http/Controllers/API/ApiController.php#L132-L134

Which means all filters are just discarded.

This PR adds in a simplified version of Statamic core's method:
https://github.com/statamic/cms/blob/5.x/src/Http/Controllers/API/UsersController.php#L39-L44

to the VerifiesPrivateAPI trait.

---

To enable filtering one can follow the steps outlined in the Statamic docs - https://statamic.dev/rest-api#enabling-filters
But make the changes specified to the config/private-api.php file instead of config/statamic/api.php

---

Setting this to draft as I plan to implement this for all the controllers unless that is not desired.

[steveparks]

I've tested this PR by applying it as a patch locally, and can confirm:

1. It fixes filtering on the /api/private/users endpoint, and results are returned accurately
2. If trying to filter on a field that hasn't been specified in config/private-api.php, it returns an error that it is a forbidden filter and doesn't return data.

This works for me!