Add resource response functionality

[mondragonfx]

Description

PR for adding return response for vultr resources to govultr.

Checklist:

• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Have you linted your code locally prior to submission?
• Have you successfully ran tests with your changes locally?

[happytreees]

This PR introduces breaking changes to the package for any endusers

re-review

[happytreees]

lgtm

[christhemorse]

Looks good after recent changes. Nothing else has jumped out at me and the approach for passing along the API json response should help enable a json feature for vultr-cli and other utilities, and give folks using the library more flexibility to use that data if they need it. Approved.