Bluetooth: HFP_AG: Support Voice recognition activation

Add configuration `CONFIG_BT_HFP_AG_VOICE_RECG` to enable Voice
recognition activation feature.

Add configuration `CONFIG_BT_HFP_AG_ENH_VOICE_RECG` to enable Enhanced
Voice Recognition Status feature.

Add configuration `CONFIG_BT_HFP_AG_VOICE_RECG_TEXT` to enable Voice
Recognition Text feature.

Add function `bt_hfp_ag_voice_recognition` activate/deactivate the
Voice recognition activation feature.

Add function `bt_hfp_ag_vre_state` to notify the state of the voice
recognition engine.

Add function `bt_hfp_ag_vre_textual_representation` to notify the
state and textual representation of voice recognition engine.

Add callback `voice_recognition` to notify the upper layer the status
of voice recognition activation.

Add callback `ready_to_accept_audio` to notify the upper layer that
the HF is ready to accept audio.

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