Remove `async_trait` in favor of stdlib

[vlopes11]

Rust doesn't support async traits as of the moment.

As soon as the feature lands in stable, we should remove async_trait from our implementation in favor of the canonical implementation.

Blocked by: https://github.com/rust-lang/rust/issues/44265

Originally posted by @adlerjohn in https://github.com/FuelLabs/fuel-bft/pull/9#discussion_r830431156