Add support for `Eio`.

[zoj613]

It would be nice to have support for using Eio as the concurrency library instead of async or lwt.

[andersfugmann]

I think it should be possible without too many foundational changes.
An EIO implementation need to provide an implementation of Aws_s3.Types.Io. I have not tried, but I had success doing this for zmq-eio implementation

'a Deferred.t should be declared as type 'a Deferred.t = 'a and then go from there.

[andersfugmann]

I've started working on this (https://github.com/andersfugmann/aws-s3/tree/andersfugmann/eio)
I've run into a few challenges:

• There does not seem to be a http streaming library for EIO that supports
• Eio needs an environment to use the network and timers (for setting timouts)