rid-macro: adding ability to skip a struct field

[thlorenz]

• introduced #[derive(rid::Config)] which adds rid config attributes to the model struct it is applied to
• #[derive(rid::Config)] itself does not add any generated code, but just serves to introduce the attrs
• those attrs are used to configure each struct field if needed, so far #[rid(skip)] to omit it from rendering FFI wrapper code
• attr syntax modeled after serde(skip)
• as with serde attrs rid(*) are extensible for future needs

NOTE: I tried to opaquely add this derive to each model struct, but caused problems when another proc_macro attribute comes after (derive attrs need to be last). In the future we could explore rewriting the TokenStream and inject the derive in the right place so the user doesn't have to add it explicitly.