[11.x] Add optional parameter for confirmed validator rule

[jwpage]

This adds an optional parameter for the confirmed validator rule, allowing for more customization.

For example, you could do "password" => "confirmed:passwordConfirmation" if your front-end coding style required camelCase.

Providing no parameter results in the current behavior of checking the default naming convention, ie. "password" => "confirmed" checks password_confirmation.

The same behavior can be achieved with "passwordConfirmation" => "same:password" but I believe this better describes the intent of the validation by associating the confirmation with the original field, along with any other password rules that may be required.

[shaedrich]

Couldn't the casing instead/additionally be a general configuration value so it could potentially used by other rules as well?

[christophrumpel]

This would be nice to have in the docs.