Add a macro to enable dynamic dispatch for `async trait`

[Tudyx]

From my understanding, as mentioned in the blog post announcing async fn in trait, this crate will support a macro to enable dynamic dispatch for async trait.

I don't want to put a pressure of any kind (I'm fine using the async_trait crate), I just think having a tracking issue will be better than polling this crate from time to time to see if it's implemented.

[Sytten]

This would be very useful for crate maintainers to be able to have a drop-in replacement for async-trait where they can enable/disable dynamic dispatch based on a feature flag.