Bluetooth: HFP_AG: Enable 3-way feature

Add a configuration `BT_HFP_AG_3WAY_CALL` to control the feature.

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

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

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

Use call object to replace AG object for callbacks `incoming_held`,
`ringing`, `accept`, `reject`, and `terminate`.

Add callback `held` to notify the call held status.

Use call object to replace AG object for function
`bt_hfp_ag_hold_incoming`, `bt_hfp_ag_reject`, `bt_hfp_ag_accept`,
`bt_hfp_ag_terminate`, `bt_hfp_ag_remote_ringing`,
`bt_hfp_ag_remote_reject`, `bt_hfp_ag_remote_accept`, and
`bt_hfp_ag_remote_terminate`.

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