Proposed Enhancements to support Salesforce Single Sign On

[ebergerson]

The upcoming Spring '21 release of Salesforce requires that all access to Pardot
be authenticated via Single Sign On with Salesforce via OAuth2. This branch provides
an approach to enhancing PyPardot4 to extend it's functionality to be ready for
the new release. This branch is NOT meant to actually be merged, as is, into
the PyPardot4 project. Rather, this pull request was issued to provide an example of an approach
and to discuss with the community if it would be a valuable addition, and if so, what changes
would be needed to prepare the code to be merged with the project.

A full description of the effort is described in the .../demo/auth_demo/README.md file

Please read the Caveats carefully and DO NOT actually merge this code.

[mneedham91]

@ebergerson thanks for this work and for making clear this is not code that should be merged :) unfortunately I cannot prioritize this project, even with the pending change. This repo was piggybacking off the work of Josh Geller and hopefully someone else is willing to take on the torch from here. If someone out there is reading this and my inactivity is getting in your way of being that person, please email me. Thanks.

[ebergerson]

@mneedham91 No problem. I understand completely. I don't have time to maintain the code either but was very interested in contributing what we learned through fixing the problem for ourselves back into the codebase. Could you advise me on the best next steps. If I decide to address the issues I raised as discussed, should I just keep it in my own repository? How should I communicate with the community to see who would like to take over the code if I improve it? If you don't have the time to answer and have to move on, just say so, and I will drop the issue here on your repo.