[REQ] Upgrade Hyper/packages in Rust Client

Created by: andrew-s

Is your feature request related to a problem? Please describe.

The hyper package used in the rust client "rust" generator is very old, sat at version 0.11 - there was a big shift in the way it works between 0.11 and 0.12. It makes sense to upgrade this to 0.14.

Describe the solution you'd like

Upgrade the package to a newer version beyond 0.12

Describe alternatives you've considered

The alternative to not upgrading is a real pain, where my application relies heavily upon hyper so I have that package within my application - to work with an older version I'd need to import hyper 0.11 and the tokio_core packages (for the handler to connect). It was easier to just use reqwest (1 extra library vs 2) however, it would be nice to not have to use any extra packages when I already have a newer hyper on hand.

Granted, I need to make these changes because I'm changing the default configuration of apis within my application, as the Configuration struct is immutable I'm unable to change it after the fact. Ideally, configuration should be decoupled from the http client library creation but, that's probably a larger task than it is to just upgrade the packages (also unsure how other packages are built and if configuration + client is a commonality)