Add demo sample

[jgrandja]

The main goal of the "demo" sample is to demonstrate usage of all the features implemented by Spring Authorization Server. The sample will also be used to address the various "How-to" guides.

• Merge custom-consent-authorizationserver into featured-authorizationserver
• Merge federated-identity-authorizationserver into featured-authorizationserver
• Merge device-grant-authorizationserver into featured-authorizationserver
• Merge device-client into messages-client
• Rename featured-authorizationserver to demo-authorizationserver
• Rename messages-client to demo-client
• Update sample README