Support standalone modules for wit file generation.

[sviezypan]

Right now in order to generate WIT file from Rust code, we annotate inner module:

#[golem_rust::create_wit_file]
mod golem_component2 {

    pub struct BidderId {
        pub bidder_id: String
    }
    trait AuctionService {
        fn create_bidder(full_name: String) -> BidderId;
    }
}

This ticket is to implement another solution if possible - to have top level attribute in a module file and having wit file generated out of it.

E.g. there's a auction_app.rs file with a top level attribute #![golem_rust::create_wit_file]. File would contain only structs, traits, enums and type aliases. Than module would be imported in main.rs with mod auction_app