[8.x] Allow custom validation rules to use prepareValuesAndOthers()

[jessarcher]

Similar to #35183, I've needed to re-implement ValidatesAttributes::prepareValuesAndOthers() (and the two other methods it calls) on a few projects when implementing custom validation rules with true/false/null parameters.

So, I was wondering whether this method could be made public?

Alternatively, I could PR my prohibited_if custom validation rule that is basically the opposite of required_if and similar to exclude_if except that it fails validation instead of excluding the field. Not sure how common the use case is though.

No hard feelings if you prefer to keep this method private - totally understandable.

[driesvints]

If we make this change we'll need to be aware it's a breaking change to the method signature.

[taylorotwell]

I renamed this to parseDependentRuleParameters and made it public. 👍