Add authentication service

[StephenHodgson]

XRTK - Mixed Reality Toolkit Feature Request

Is your feature request related to a problem? Please describe

Adding an authentication service that utilizes OAuth flows from any OAuth provider would enable developers to add simple and secure user authentication. All data providers should be implemented by the OAuth provider to ensure maximum security. We would only be adding a unified interface with access to the usual APIs.

How would you classify your suggestion

• Usability / Configuration
• Architecture / Services

Describe the solution you'd like

• Creation of an authentication service.
• Creation of OAuth data providers from Steam, Google, Microsoft, Facebook, and Apple. (Additional data providers can be added or implemented as needed)
• Where possible support multi factor authentication
• Where possible support code flow login workflows
• Where possible support QR code flow login

Additional context

• We're not to compromise security or implemented our own OAuth workflows.
• We will not support username and password flows