#[async_trait]Expand description
The #[async_trait] attribute, re-exported so an out-of-crate implementor of
RequestPolicy (or http::auth::HttpAuthProvider) does not have to add an
async-trait dependency of its own — and, more importantly, cannot end up on a
DIFFERENT version of it than the trait was declared with, which produces a
signature-mismatch error that reads as a lifetime bug.