Allow step executor args step to use a data model

[ibelar]

Make it possible to use a Data model in order to setup an action arguments.

Advantage:

• args can be define as Model field;
• args input can be validate using a model Validator;

[mvorisek]

Atk4/data UA must be improved to validate the $args againts the $ua->args definition (which needs also an improved phpdoc).

https://github.com/atk4/data/blob/06dc74732a/src/Model/UserAction.php#L144

Then these validations:

• https://github.com/atk4/ui/blob/8926412a31/src/UserAction/BasicExecutor.php#L71
• https://github.com/atk4/ui/blob/8926412a31/src/UserAction/BasicExecutor.php#L100
• https://github.com/atk4/ui/blob/8926412a31/src/UserAction/JsCallbackExecutor.php#L115

can be removed which are even wrong, as isset() will fail with null value like empty date.