DMN: provide better support for boolean-like values

[sergei-maertens]

Currently to get a boolean passed to the DMN evalution you can only do so by using a checkbox widget (or a JSON logic rule perhaps that translates something else into a boolean).

Using a radio widget with values "true" and "false" should have an option to "cast" this value to a boolean type (probably in the DMN action configuration?).

The Camunda API call can interpret a string value "true" correctly if we provide the type: Boolean information, but now our backend sends type: String.