Make sdk http-client agnostic

[sosthene-nitrokey]

I wonder if it would make sense to implement the operations in terms of the http crate instead of using ureq directly. This would allow us to make ureq optional and ureq updates would no longer be breaking changes. We could of course still provide a default implementation using ureq.

http is also supported by hyper and reqwest so we would cover all major Rust HTTP clients.

We could either let the user provide something like a Fn(http::Request) -> Result<http::Response, Box<dyn Error>> as part of the Configuration. Or provide structs that can be converted to and from the http types to implement the operations – that would even be compatible with async HTTP clients.

Originally posted by @robin-nitrokey in #35 (comment)