Add proxy server support

[eosmith]

Description

Add http proxy server support

Motivation and Context

These updates allow a Client to be created that routes requests through an http proxy server. This will resolve issue #12

How Has This Been Tested?

This was tested using our organizations proxy server to successfully connect to an application created in our Duo account. A new test was not added as connecting through a live proxy server is organization dependent.

Types of Changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

[eosmith]

This looks really good! Thank you so much for adding proxy server support this this repo. I am sure a lot people would appreciate this feature too!

You're welcome and thank you so much for your review and help to get this added!