Bluetooth: AVCTP: Implementation of AVCTP.

This patch implementing avctp.c
New Kconfig BT_AVCTP is provided to enable this layer.
avctp_internal.h shows the APIs for the upper layer, i.e., AVRCP.

Only connection and disconnection interfaces are provided in this patch.

Signed-off-by: Zihao Gao <gaozihao@xiaomi.com>