feat(bitreq): add sans i/o support

[oleonardolima]

I've copied this issue from: tcharding/minireq#4, will try to work on this after upgrading rust-esplora-client.

It would be nice if users could use this crate with no I/O at all.

Consider adding an API that uses buffers only. Users would then need to fill and drain the buffers themselves.

[tcharding]

Yes, this feature is in demand.