Add the ability to specify Services instead of Classes for GridFieldAddNewMultiClass

[NightJar]

silverstripe-gridfieldextensions/src/GridFieldAddNewMultiClass.php

Lines 216 to 221 in 856fce1

	if (!$class || !array_key_exists($class, $classes)) {
	throw new HTTPResponse_Exception(400);
	}
	$unsanitisedClass = $this->unsanitiseClassName($class);
	$handler = Injector::inst()->create(

This check could easily become Injector::inst()->has( instead of class_exists(

And work will need to ensure compatibility with filtering that happens in getClasses:

silverstripe-gridfieldextensions/src/GridFieldAddNewMultiClass.php

Lines 135 to 138 in 856fce1

	if (!class_exists($class)) {
	continue;
	}
	$is_abstract = (($reflection = new ReflectionClass($class)) && $reflection->isAbstract());

Note that this may need some effort to get GridFieldAddNewMultiClassHandler to play nice.