Bluetooth: HFP_HF: handle unsolicited result code +VGS/+VGM

Handle the unsolicited result code +VGS and +VGM if the configuration
`CONFIG_BT_HFP_HF_VOLUME` is enabled.

Add a callback `vgm` to notify the application speaker microphone
notification if the configuration `CONFIG_BT_HFP_HF_VOLUME` is enabled.

Add a callback `vgs` to notify the application speaker gain
notification if the configuration `CONFIG_BT_HFP_HF_VOLUME` is enabled.

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