Allow settings to be updated and read.

[spacedmonkey]

Working on #48885. Decided to go a different direction than the ticket name. Use the existing endpoint but add some new callbacks.

To use this PR, here is the example I use to test.

add_action('init', function(){
	register_setting(
		'general',
		'wibble',
		array(
			'show_in_rest' => array(
				'name' => 'wibble',
				'edit_permission_callback' => '__return_true',
				'read_permission_callback' => '__return_true',
			),
			'type'         => 'string',
			'description'  => __( 'Site title.' ),
		)
	);
});

This make a setting called wibble, that will now be public to read and edit. You never do this, but it gives you an idea of how the code works.

Trac ticket: https://core.trac.wordpress.org/ticket/48885

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