Bluetooth: HFP_AG: Attach a phone number for a voice tag

Add configuration `CONFIG_BT_HFP_AG_VOICE_TAG` to enable the feature
`attach a phone number for a voice tag`.

Handle AT command `AT+BINP=1`.

Add callback `request_phone_number` to notify the request and get the
phone number from the upper layer.

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