Allow for custom search functionality in select fields

see : https://github.com/romainmenke/acf/pull/1

This allows for custom search functionality.

We sometimes use the select fields with dynamic data in combination with an external API. This works great!

If the API supports search it makes sense to forward $_POST['s'] to this API. The build in filter however does a very simplistic stripos validation on the label of the choices. A more sophisticated search (API-side) might include an item because it matched all terms found in $_POST['s'] but in a different order.

This change allows implementers to modify or disable the build in filter on $_POST['s'].

Examples :

// disable acf filter on $_POST['s']
add_filter(
	'acf/fields/select/search/name=<my field>',
	function( $include, $args, $field, $post_id ) {
		return true;
	},
	10,
	4
);

// look for individual words by splitting on spaces in $_POST['s']
add_filter(
	'acf/fields/select/search',
	function( $include, $args, $field, $post_id ) {
		$label = $args['label'];
		$s = $args['s'];

		$terms = explode(" ", $s);
		foreach ($terms as $term) {
			if (stripos($label, $term) !== false) {
				return true;
			}
		}

		return false;
	},
	10,
	4
);

Start at the select-field search handling that applies the built-in stripos filter to $_POST['s'], then review the referenced pull request for the proposed hook behavior. Done means implementers can modify or disable that filter through the documented acf/fields/select/search hooks and the supplied examples remain valid.