Add support for Client Credentials Grant Type to OAuth2 authentication

Add support for Client Credentials Grant Type to OAuth2 authentication OAuth2 support requires that only tokens associated with users may be used with authenticated resources. However, OAuth2 also support a client credentials grant type which does not require any user interaction. This is typically used directly by applications to access such API. This does not work with RESTful as once a resource is marked as authenticated, there has to be an associated user. This change allows plugin developers to specify a user which will be used when a client credentials token is used. The user may be specified with the uid or name. The uid has the higher priority.