DPP: Move Authentication Response building into a separate function

DPP: Move Authentication Response building into a separate function

This cleans up old dpp_auth_build_resp() (now dpp_auth_build_resp_ok())
a bit by separating initialization steps for a DPP authentication
session from the code needed to build the frame. This allows
dpp_auth_build_resp_status() to share the helper function instead of
having to maintain a duplicated message construction implementation. In
addition, this makes it easier to remove some of the attributes for
protocol testing purposes.

Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>