add GUEST_READ_WRITE to JWT scope, for guest users

See easybib/issues#831

This is a change in the builder, and is very difficult to test, due to the very nature of the builder, i.e. to "hide" all the setup from the user of api-client-php. It would require adding an interface to the builder to inject stuff through the builder into the `JsonWebTokenGrant\TokenRequestFactory`; IMO we can leave this change without a test.