Bluetooth: HFP_AG: Improve codec negotiation

Add a configuration BT_HFP_AG_CODEC_NEG to configure the feature.

Add a function bt_hfp_ag_audio_connect to setup the codec negotiation
procedure.

Improve AT+BAC and AT+BCC handle.

Add callback codec_negotiate to notify the application of codec
negotiation result.

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