[Enh]: Introduce new authentication provider `unauthenticated`

[JerryNixon]

What?

A new authentication provider, Unauthenticated, where all operations run as anonymous.

Why?

When Data API builder is behind an app gateway or APIM, authentication is often handled there.

This provider is similar to AppService but skips any use of HTTP authentication headers.

Command Line

dab configure --runtime.authentication.provider Unauthenticated

Rules

1. This provider is allowed even when permissions are configured for authenticated or custom roles.
2. This provider is allowed even if host.mode is set to production.