Tokio 1.0 support

[niklasad1]

Hey, thanks for this library and I would like to see support for tokio 1.0, for example paritytech/jsonrpsee#227 would help us to move to surf instead of directly using hyper.

Currently this seems to blocked on http-rs/http-client#74

Is there anything else that is blocking and what can I do to help to push this forward?

[sunng87]

Once http-rs/http-client#74 merged we can simply upgrade async-std to 1.9 and use their tokio1 feature to get surf work on tokio 1.0.

[Fishrock123]

semver-major because we re-export http-client stuff and it's also semver-major over there

(And would also be semver-major if it were all here.)