Convert from/into web_sys::Request/Response

[olanod]

It's nice that http-types compiles to WASM in the browser, even nicer would be to have a way to convert back and forth between the Request/Response types in this crate and the "native" JS Request/Response to avoid wrapping with a new type. The feature can be gated with a flag(e.g. web).
This is probably better and more general purpose than what I proposed in http-rs/http-client#60 and can eventually simplify the code over there. I also think conversion could be made more efficient if the JS Request/Response body that is a ReadableStream(implements the asyncIterator protocol) is converted to a Stream/AsyncReader, modern Chrome even allows transferring those streams with the postMessage API so for my use case of intercepting requests from a ServiceWorker to be handled in a Worker(Using Rust and http-types) could be made much more efficient in the future.