[0.5.x] Improve async validation API

[timacdonald]

The API when working with a form wizard is less than ideal. We have done a lot of work recently to make this experience better, but the API never felt right.

You currently need to call the touch method to ensure the current page's inputs are validated - even if the user has not interacted with them.

<button
    onClick={() => form.touch(['name', 'email']).validate({
        onSuccess: () => nextStep(),
    })}
>
    Next Step
</button>

It is also not possible to un-touch an input, which can cause issues with certain form wizard configurations.

This PR intends to make this API better while addressing the issue with not being able to un-touch inputs.

To achieve this we are exposing an only validation key as part of the documented public API. This configuration value already exists, but it is currently called validate and is not intended for public use.

I'm renaming the key to only now that it is public, which makes it a more expected API name in the Laravel ecosystem. I've depreciated validate but kept support for it in case people have been using it.

<button
    onClick={() => form.validate({
        only: ['name', 'email'],
        onSuccess: () => nextStep(),
    })}
>
    Next Step
</button>

When specifying inputs in the only key, validation will only apply to those inputs. Those keys will also be "touched" to ensure their future validation behaviour is expected, including conditional and interdependent validation rules.

[timacdonald]

Docs: laravel/docs#10058

[timacdonald]

Marking this one as draft while I consider how this API naming could conflict with with Inertia's visit options.