Use `type` attribute to determine conversion target type instead of field type

[remkop]

This allows fields to be declared as interface or abstract types, and picocli can still convert string command line args to the concrete implementation class specified in the type attribute.

Example:

class App {
    @Option(names = "--num", type = BigDecimal.class) // subclass of field type
    Number[] number; // array type with abstract component class

    @Parameters(type = JapaneseAddress.class) // concrete impl class
    IAddress address; // type declared as interface
}

If the type attribute is not specified, fall back to the array component class or (for single-value fields) the field type.