[feat] Add database constraints for validation annotations

[Klaas058]

Currently when using annotations with database rules like Exists and Unique, it is not possible to include database constraints.

This will throw a php error:

// Error: Constant expression contains invalid operations
#[Exists(Customer::class, column: 'id', where: fn($query) => $query->where('active', true))]
public ?int $customer_id

This feature would allow using database constraints like so:

// Valid php 🎉
#[Exists(Customer::class, column: 'id', where: new WhereConstraint('active', true))]
public ?int $customer_id

After testing this has been working great for us, happy with any feedback

One thing I'm not a 100% sure about:

• Types in the DatabaseConstraint implementations are mixed since the Laravel methods are untyped themselves. Is this the best way?

[rubenvanassche]

Looking great, few changes required and then this one is good to go!