OAuth2 Support

[bstopp]

Description

Update API to support OAuth Server-to-Server client credentials approach to Authentication.

Related Issue

• 155

• 163

Motivation and Context

Updates library to prepare for removal of deprecated JWT solution.

How Has This Been Tested?

Update existing unit tests, and added new ones.

Screenshots (if appropriate):

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)

Checklist:

• [x] I have signed the Adobe Open Source CLA.
• [x] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [x] I have added tests to cover my changes.

[francoisledroff]

@bstopp
I scanned this PR quickly today... it's looking really good , thanks ! I need to test this change further in my own little projects ... I'll get back to you by the end of the week

[bstopp]

Near as I can tell, this is ready to go. I'm updating the CloudManager library based on these changes.

[bstopp]

Ok, now it should be good to go.

[rperelma]

@francoisledroff will you merge it?