Add Oauth 2.0 Client Credentials Flow

We want to add the Client Credentials Flow to the existing OAuth 2.0 flows implemented with the Nuxeo Server.

The Client Credentials Flow involves an application exchanging its application credentials, such as client ID and client secret, for an access token.

This flow is best suited for Machine-to-Machine (M2M) applications, such as CLIs, daemons, or backend services, because the system must authenticate and authorize the application instead of a user.

(from https://auth0.com/docs/get-started/authentication-and-authorization-flow/client-credentials-flow)

See OAuth 2.0 RFC 6749, section 4.4