Bluetooth: HFP_HF: Support HF Indicators

Add configuration `CONFIG_BT_HFP_HF_HF_INDICATORS` to enable feature
HF Indicators.

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

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

Add function `bt_hfp_hf_enhanced_safety` to transfer enhanced safety
status.

Add function `bt_hfp_hf_battery` to transfer Remaining level of
Battery.

Send AT command to notify AG the supported HF indicators of HF in SLC
initialization sequence.

Send AT command to get the supported HF indicators of AG in SLC
initialization sequence.

Send AT command to get the enabled/disabled state of generic status
indicators from AG in SLC initialization sequence.

Handle unsolicited result code `+BIND`.

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