Feature: Ability to inject any PSR-7 HTTP Client

[bmeynell]

It would be valuable to be able to inject a specific PSR-7 HTTP Client, for example, guzzlehttp, instead of relying on the custom curl implementation that currently underlies all the API calls. This would allow users to reliably inject any customized HTTP client (because of the standardized interface) that meets their specific requirements while still being able to take advantage of the API calls provided by this library.

[thomasschiavone]

Thanks again for flagging. Will also get this in front of the team.