[rust client] Feature: Generic requests–agnostic of transportation or tokio

Created by: colelawrence

Description

For my own application, I just finished updating the rust-client implementation from using hyper 0.11 to using hyper 0.12 (see diff of conversion)

I thought that I had to do this in order to be able to use the client generator with my generated rust-server, because all of the current examples are suited for hyper 0.12 or newer versions of tokio/futures.

openapi-generator version

3.1.0 accretion (no regression)

Suggest a fix/enhancement

This is where my idea comes in. I would like to be able to choose my own transport and not worry about conflicting threads or things like Send and Sync as a beginner. I would like to be able to leverage a client generator for constructing requests against the correct endpoint with the correct headers, but remove it's dependence on specific async runtimes.

I would like the ::apis::Request struct to implement a generic request which I could impl Into<hyper::Request> for event_service_http::apis::Request {} for whichever version of hyper, reqwest, actix-* http library the library is using.

We could enable hyper specific client using cargo features, I think.

What are your thoughts? @bjgill @euank @wing328