Review and changes from Jocelyn Fiat

a solution to manage authentication is to check the authentication before anything, then store the result into WSF_REQUEST.execution_variables.
That's why is done here, so that we can pass the authenticated username by param, or also get it from the WSF_REQUEST object itself from anywhere, as it is done in `handle_other' for instance.

So the auth challenge is presented only for /login uri
and then for other uri, the code just check if there is an authenticated user or not.

My Current patch is not that great related to digest information that is also stored in execution_variables.
Maybe we could just store the HTTP_AUTHORIZATION directly.

After in real application, this should be handle in a generic way, but that's out of the scope for now