Provide conversion function for reqwest::Response to http::Response<_>

[jplatte]

Original title: Provide conversion functions for reqwest::{Request, Response} to http::{Request<>, Response<>}

The reverse already exist as one From (response) and one TryFrom (request) trait implementation, but going from reqwest types to http types doesn't seem to be supported yet.

[vasilakisfil]

I came to this repo to post the same issue. I fully support that, it would remove a lot of boilerplate.

[jplatte]

Ping @seanmonstar, are you interested in a PR for this?

[seanmonstar]

Sure, makes sense to me!

[jplatte]

@seanmonstar I think this should not be marked easy, at least if you want the remaining Response conversion to work on WASM too.