Abilities API: Add filters for input and output validation

Introduce the `wp_ability_validate_input` and `wp_ability_validate_output` filters so developers can layer custom validation on top of the default JSON
Schema checks, either augmenting an existing WP_Error or rejecting otherwise valid data.

Props priethor, gziolo, westonruter, enej.
Fixes #64311.



git-svn-id: https://develop.svn.wordpress.org/trunk@62398 602fd350-edb4-49c9-b593-d223f7449a82