Bluetooth: HFP_AG: Support HF Indicators

Add configuration `CONFIG_BT_HFP_AG_HF_INDICATORS` to enable feature
HF Indicators.

Add configuration `CONFIG_BT_HFP_AG_HF_INDICATOR_ENH_SAFETY` to
support HF indicator `Enhanced Safety`.

Add configuration `CONFIG_BT_HFP_AG_HF_INDICATOR_BATTERY` to support
HF indicator `Remaining level of Battery`.

Add function `bt_hfp_ag_hf_indicator` to activate/deactivate HF
indicator.

Optimize the handle of AT command `AT+BIND`.

Handle AT command `AT+BIEV`.

Add callback `hf_indicator_value` to notify the value of HF indicator.

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