[8.x] Allow caching to be disabled for virtual attributes accessors that return an object

[decnorton]

Context: #40497

This PR allows caching to be disabled for virtual attributes being accessed used the new Attribute class method.

I often find myself adding virtual accessors to construct objects based on other attributes within the model, most commonly DateTime instances. Currently, the result of an accessor is cached if it's an object and only invalidated if the attribute's mutator is called. Scalar types are unaffected.

This PR adds a flag that will disable this behaviour. Mutating multiple attributes & value object casts will be unaffected by default.

Example

public function finish(): Attribute
{
    return Attribute::getWithoutCaching(fn () => $this->start->addSeconds($this->duration)));
}

or

public function finish(): Attribute
{
    return Attribute::get(fn () => $this->start->addSeconds($this->duration)))->disableObjectCaching();
}

[taylorotwell]

Thanks for contributing to Laravel! ❤️

[RoodFruit]

@decnorton @taylorotwell shouldn't this be the default behavior of accessors (virtual attributes)? Why are they cached in the first place?