[11.x] feat: add generics to eloquent Attribute cast

[calebdw]

Hello!

This PR adds generics to the Eloquent Attribute cast.

Thanks!

[taylorotwell]

Hello, please resubmit with before and after screenshots of the incorrect behavior in your IDE before and correct behavior after.

[calebdw]

@taylorotwell,

Larastan uses these generics to determine the readable (TGet) and writable (TSet) property types for the attributes. I was trying to move this stub into the framework

Best,