Bluetooth: HFP_AG: Send subscriber number info

Handle AT command `AT+CNUM`.

Add a callback function `subscriber_number` to notify the upper layer
that the HF needs to get subscriber number information from the AG.

If there is subscriber number information needs to be sent, the passed
function `bt_hfp_ag_query_subscriber_func_t func` can be called for
this purpose.

Signed-off-by: Lyle Zhu <lyle.zhu@nxp.com>