Enable Asynchronous Handlers

[Kab1r]

It would be nice if we could have an asynchronous handler for a function. I know you can do this with the Node API.

Ideally, I should be able to pass a handler function that returns a future into the lambda macro.

Alternatively, there could be a separate macro for asynchronous handlers.

[mike-engel]

@KabirKwatra I'll take a look at this when I get a chance, but if you have ideas, PRs are welcome

[srounce]

awslabs/aws-lambda-rust-runtime supports this already on the master branch but they haven't cut a release because reasons. The current master branch API has changed since 0.2 but any changes that may need to happen minimal, the API surface of now_lambda should be unaffected.

[leerob]

Yay, this is supported now!

https://github.com/awslabs/aws-lambda-rust-runtime/releases

[ecklf]

Hey we released a new runtime in #80 which should now enable this. Happy about every feedback.