eap_proxy: Add support for SIM state change indication from eap_proxy

eap_proxy: Add support for SIM state change indication from eap_proxy

This registers a new callback to indicate change in SIM state. This
helps to do some clean up (more specifically pmksa_flush) based on the
state change of the SIM. Without this, the reconnection using the cached
PMKSA could happen though the SIM is changed.

Currently eap_proxy_sim_state corresponds to only SIM_STATE_ERROR. This
can be further extended.

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