DPP: Support retrieving of configurator's private key

DPP: Support retrieving of configurator's private key

To retain configurator information across hostapd/wpa_supplicant
restart, private key need to be maintained to generate a valid pair of
authentication keys (connector, netaccess_key, csign) for new enrollees
in the network.

Add a DPP_CONFIGURATOR_GET_KEY control interface API through which the
private key of an existing configurator can be fetched.

Command format:
DPP_CONFIGURATOR_GET_KEY <configurator_id>

The output from this command can then be used with
"DPP_CONFIGURATOR_ADD key=<hexdump>" to create the same key again.

Signed-off-by: Jouni Malinen <jouni@codeaurora.org>