add LE Data Channel Events (L2CAP_EVENT_LE_*). implement can send now logic

use ENABLE_LE_DATA_CHANNELS, add compile flag to doc

validate security level on incoming le data channel connection requests

require hci connection for l2cap le data channels

l2cap: log error if channel not found

l2cap: extend test, fix bugs, add missing checks to pass PTS

l2cap: count credits with le data channels. implement auto credits

l2cap: re-assemble le data channel SDUs

l2cap: fragment outgoing le data channel SDU

l2cap: implement le disconnect

l2cap: send/receive le data channel SDUs that fit into single packet

l2cap: close l2cap le connection on hci disconnect

l2cap: send and receive le connection response accept

l2cap: send and handle le connection decline

l2cap: emit conn request on le data con request, handle le accpet/decline

l2cap: extract l2cap_le_signaling_handler_dispatch

l2cap: handle hci le connect, send le credit based connection request

pts: le_data_channel sets up hci connection

Merge branch 'develop' into le-data-channel

l2cap: drop reason arg for l2cap_decline_connection - only 'no resources' expected

l2cap: return error for l2cap_unregister_service

l2cap: start creating connection via gap whitelest

l2cap: extract l2cap_create_channel_entry. implement register/unregister le service

l2cap: avoid potential null dereference in l2cap_rtx_timeout

l2cap: correct list of fixed channels supported