Provide a Shorthand for adding Send bounds

Hey everyone,

it would be great if we could have a shorthand for making a trait Send only, like this:

#[trait_variant::only_send]
pub trait SomeTrait {
  async fn hello() -> i32;
}

desugaring to

pub trait SomeTrait: Send {
  fn hello() -> impl Future<Output = i32> + Send;
}

(no second variant)

The rationale behind it being that with the mostly Multithreaded Executors that are used widely right now (like tokio), this will probably be the default case once AFIT is becoming stable on 28th.

No files or tests are named. Start by reading the repository's existing trait_variant handling and compare it with the requested #[trait_variant::only_send] syntax and desugaring; done means the shorthand produces a single Send-bound trait variant with the stated async method behavior and has coverage for that transformation.