Add support for ASYNC

[Lykos153]

https://git-annex.branchable.com/design/external_special_remote_protocol/async_appendix/

[yarikoptic]

Hi, I wonder if implementing support for ASYNC is on the roadmap? Asking because I know that @mih et al would be interested as well, and it might motivate me to look into reimplementing our elderly customly coded custom remotes in datalad to just use annexremote as well (instead of just implementing async support in there without RF).

[Lykos153]

Yes, that's the next thing I'm going to spend my time on after fixing this google issue!