Add and validate samples for using the cli in an unattended workflow

[samwelkanda]

Overview

Add authentication samples for customers will want to use the CLI without a user. Users can authenticate an application using confidential client flow with a client secret or a certificate thumbprint

Demo

Notes

Optional. Ancillary topics, caveats, alternative strategies that didn't work out, anything else.

Testing Instructions

• Register an application in Azure AD to connect to Microsoft Graph
• Generate a client secret
• Or generate certificate and update the Azure Active Directory Application with the Certificate
• Pull changes on this branch
• Activate your virtual environment
• Run ./mgc login --service-principal -u <APPLICATION/CLIENT ID> -t <TENANT ID> -p <CLIENT SECRET/CERTIFICATE THUMBPRINT>

Fixes #50

Microsoft Reviewers: Open in CodeFlow

[samwelkanda]

Since the move to azure cli core, only command modules are checked into the repo. Beta extensions are downloaded on a need be basis from cloud storage.