Allow creation of async variant for non-async triats

Not sure if this was considered when designing the crate, but I was hoping to have the following use case as handled by this crate as well

#[trait_variant::make(async_variant = [LocalAsyncSomeTrait, AsyncSomeTrait: Send])]
trait SomeTrait {
    #[variant(make_async)]
    fn blocking_method(&self) -> Result<Value, Error>;

    #[variant(inherit)]
    fn method_to_auto_impl(&self);

    #[variant(skip)]
    fn method_is_skipped(&self);
}

The above would expand to

trait LocalAsyncSomeTrait: SomeTrait {
    fn blocking_method(&self) -> impl Future<Output = Result<Value, Error>>;

    fn method_to_auto_impl(&self) {
         <Self as SomeTrait>::method_to_auto_impl(self)
    }
}

trait AsyncSomeTrait: LocalAsyncSomeTrait {
    fn blocking_method(&self) -> impl Future<Output = Result<Value, Error>> + Send;

    fn method_to_auto_impl(&self) {
         <Self as SomeTrait>::method_to_auto_impl(self)
    }
}

This way a single crate can defined multiple traits. This is in no way refined and probably needs some design, but would be great addition in my opinion

No source files or tests are named. Start by reviewing the existing trait_variant macro entry point and how it expands async variants, then compare that behavior with the proposed make_async, inherit, and skip attributes. Done means the design is settled and the requested local and Send async traits are generated consistently.