Bluetooth: HFP_HF: Enable 3-way feature

Add a configuration `BT_HFP_HF_3WAY_CALL` to control the feature.

Add a configuration `BT_HFP_HF_MAX_CALLS` to define supported maximum
calls.

Add a configuration `BT_HFP_HF_ECS` to configure the Enhanced Call
Status feature.

Add a configuration `BT_HFP_HF_ECC` to configure the Enhanced Call
Control feature.

Add a structure `struct bt_hfp_hf_call` to manage the call.

Add the call object to callback `outgoing`, and`incoming`.

Use call object to replace AG object for callbacks `remote_ringing`,
`on_hold`, `accept`, `reject`, `terminate`, `call_held`,
`ring_indication`, and `clip`.

Change callback `call_held` to `held`.

Add callback `retrieve` for the retrieved held call.

Add callback `call_waiting` to notify the application a new call is
waiting.

Use call object to replace AG object for function `bt_hfp_hf_accept`,
`bt_hfp_hf_reject`, `bt_hfp_hf_terminate`, and
`bt_hfp_hf_hold_incoming`.

Add functions for 3-way feature, including
`bt_hfp_hf_call_waiting_notify`, `bt_hfp_hf_release_all_held`,
`bt_hfp_hf_set_udub`, `bt_hfp_hf_release_active_accept_other`,
`bt_hfp_hf_hold_active_accept_other`, `bt_hfp_hf_join_conversation`,
`bt_hfp_hf_explicit_call_transfer`, `bt_hfp_hf_release_specified_call`,
and `bt_hfp_hf_private_consultation_mode`.

Add a new function `at_get_raw_string` to get the raw string from AT
response.

Move definitions of AT+CLCC value from hfp_ag_internal.h to
hfp_internal.h.

Monitor unsolicited result code `+CCWA`.

Enable `Call Waiting Notification` if the feature is supported by both
side when the SLC connected.

Get CHLD values supported by AG if the 3-way calling feature is
supported by both side.

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