Support registered settings

[jkmassel]

WordPress Core has the register_setting API that allows developers to register custom settings that can be used throughout the WordPress UI.

These are strongly(ish) typed – the schema can be viewed with curl localhost/wp-json | jq '.routes["/wp/v2/settings"].endpoints[1].args'.

We should support the arbitrary key/value pairs that can be fetched here – the defaults will always be available (you can rg register_setting to see all the places in the codebase where settings are registered – any that have show_in_rest => true` will be part of the API response.

This trivial plugin shows an example of a custom setting – dropping it in wp-content/mu-plugins/index.php will add a custom element to the response:

<?php

function register_settings_for_test_plugin() {
  register_setting('my_stuff', 'do_the_thing', [
    'type' => 'boolean',
    'label' => "Should it do the thing?",
    'description' => "",
    'show_in_rest' => true,
    'default' => false
  ]);
}

add_action( 'init', 'register_settings_for_test_plugin' );