Support dynamic #[values]

[tomasol]

It would be nice to provide the list of values by a function:

Example:

#[derive(strum_macros::EnumIter, Debug, PartialEq)]
enum Color {
    Red,
    Green,
    Blue,
}
#[rstest]
#[test]
fn test(#[values_dyn( Color::iter() )] value: Color) {
..

[la10736]

As I already mentioned in lot of tickets about this topic (#29 is just one of them), Rust is not Python. When the #[rstest] procedural macro is processed we don't know anything about Color type because we are in an early stage where just the abstract syntax tree was build so we cannot process Color::iter() to generate the 3 test functions to compile.