rbacLogin() - accept pre-generated token by kwarg

Allow to authenticate to pyJapc by passing in an existing token, which will be set to the Java world (static token holder).

In principle the functionality already existed, but could only read a b64 encoded token from the environment. This adds an optional kw argument to pass in an existing token (either a bytes-like object, or something the returns a bytes-like object on encode() like the PyRbac token - a protocol to avoid an explicit dependency).