Replace &'a Ident with Cow in FnArg Enum

[newcomertv]

Deriving PyClass for Tuple Enums requires defining identifiers for anonymous fields.
In PR: #4072 this is done by leaking memory during the build process.

FnArg takes a reference with lifetime which would need to outlive the scope of the derive macro where the ident is created.
In order to deallocate this properly we need to move ownership into FnArg for that case.