Should non-boolean options with arity=0 be disallowed?

[remkop]

(This is a follow-up to #509)

For boolean options ("flags") it makes sense to have arity = "0", meaning, the option takes no parameters. Other types commonly need a value.

This ticket is to determine whether there should be validation in the model to prevent application authors from defining options like the below:

@Option(names = "-x", arity = "0") int x;

[remkop]

The answer is no, this should not be validated.

When the work on #536 is complete, arity = "0" will be the default value for interactive options and positional parameters: by default interactive options will not consume an argument from the command line, but instead prompt the user for input. (After #536 is complete, interactive options may be defined with arity = "0..1" to allow optional password values as command line arguments.)