REST API: Support the multipleOf JSON Schema keyword.

Props yakimun.
Fixes #51022.


git-svn-id: https://develop.svn.wordpress.org/trunk@49063 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/tests/rest-api/rest-test-controller.php

@@ -46,6 +46,7 @@ class WP_REST_Test_Controller extends WP_REST_Controller {
 				),
 				'someinteger'    => array(
 					'type'             => 'integer',
+					'multipleOf'       => 10,
 					'minimum'          => 100,
 					'maximum'          => 200,
 					'exclusiveMinimum' => true,