Fix #[cgp_getter] macro when the getter trait contains generic parameters

[soareschen]

Summary

This PR fixes the incorrect derivation of #[cgp_getter] when the getter trait contains generic parameters. Previously, the getter trait did not allow generic parameters to be specified, but this limitation was lifted in #[cgp_auto_getter]. So we also need to update #[cgp_getter] to better support generic parameters.

Example

With the new support, we can use #[cgp_getter] with traits such as follows:

#[cgp_getter {
    provider: NameGetter,
}]
pub trait HasName<App>
where
    App: HasNameType,
{
    fn name(&self) -> &App::Name;
}