Bluetooth: HFP_HF: Support phone number calling

Add a function `bt_hfp_hf_number_call` to initiate outgoing voice calls
by providing the destination phone number to the AG.

Add a callback `dialing` to notify the application the result from AG.

Add a flag `BT_HFP_HF_FLAG_DIALING` to make sure there is only one
dialing call at the same time.

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