Support async interface

[ikrivosheev]

Hello! Thank you for the great library! I want to add async interface. Is it ok?

[michaelklishin]

@ikrivosheev I cannot comment on such a terse problem definition.

The biggest headache around supporting an async API is the dependencies. So it will have to be an opt-in feature using, for example, some Tokio bits.

Otherwise I have no objections, we have HTTP API clients that support two versions and it was the goal with the namespacing used here.

[michaelklishin]

I see that in #26, even the blocking client becomes optional. In which case making the dependency-heavy async one optional is probably what you had in mind anyway.

[ikrivosheev]

I see that in #26, even the blocking client becomes optional. In which case making the dependency-heavy async one optional is probably what you had in mind anyway.

Yes, you are right. I meant async client would be optional