[8.x] Add gate policy callback

[cbl]

This pr adds the ability to pass policy callbacks to the gate.

Usage

Gate::inspect([PostPolicy::class, 'update'], $post)->authorize();
Gate::inspect(InvokablePolicy::class)->authorize();
Gate::check([PostPolicy:class, 'update'], $post);
Gate::any([PostPolicy::class, ['update', 'edit']], $post);

Why This Is Usefull

• This allows navigating to the namespace of the policy from where it is used.
• Different policies can be used for the same model.

[taylorotwell]

Can you look at the AuthorizesRequests trait? It does some argument parsing before calling into the gate and it may need to be updated to support these changes.

I will mark this PR as draft until that is done - please mark it as active when you are ready for me to review it again.

[cbl]

@taylorotwell I changed the trait so the it works when an array is passed to the authorize method. However calling the __invoke method when only passing a class name would introduce a breaking change as a different method name is guessed by the trait.

[driesvints]

Reverting this. See #39267