Add custom casters to caster factory

[brendt]

Add the ability to add custom casters for specific types to the caster factory. One common example is Carbon, which should have a CarbonCaster. Instead of having to add #[CastWith] on every Carbon property, you'd like a package or project to define it once. Something like:

$casterFactory->addCaster(Carbon::class, CarbonCaster::class);