Add support for anonymous classes as model observers

[Persaeus]

Good day Laravel community!

As you know, a model observer can be registered like so:

use App\Models\User;
use App\Observers\UserObserver;

User::observe(UserObserver::class);

---

In the DocBlock of the relevant registerObserver() method, it is indicated that, aside from using class strings, observers may also be registered using instantiated objects (@param object|string $class)...

framework/src/Illuminate/Database/Eloquent/Concerns/HasEvents.php

Lines 47 to 55 in 0bb3d76

	/**
	* Register a single observer with the model.
	*
	* @param object|string $class
	* @return void
	*
	* @throws \RuntimeException
	*/
	protected function registerObserver($class)

This works fine for regular, instantiable objects for which a class can be determined.

However, it breaks (silently) when trying to register an observer object via an anonymous class, as these cannot later be instantiated...

User::observe(new class() {
    // listener methods...
})

The same goes if we try to register an observer object that depends on properties that cannot be arbitrarily instantiated...

$observer = new UserObserver;

$observer->logEvents = true; // has no effect

User::observe($observer);

Adding support for anonymous objects and stateful objects as observers should be reasonably straightforward, as we already support closure-based event listeners, and these can be constructed from any object.

For anonymous objects, this could be perfectly backwards-compatible, as these aren't currently supported at all.

With stateful objects, it's a little more complicated, as users may depend on the current implementation. But it may be worth looking into for an upcoming major version.

Anyone else think this is worth looking into?

If this idea gets green-lighted, I am willing to get the ball rolling with a PR with tests.