MINOR: event_hdl/proxy: publish EVENT_HDL_SUB_PROXY_{ADD,DEL,PUB,UNPUB} events

MINOR: event_hdl/proxy: publish EVENT_HDL_SUB_PROXY_{ADD,DEL,PUB,UNPUB} events

Proxy events are published in the per-proxy subscription list as well as
global one. EVENT_HDL_SUB_PROXY_ADD is published when proxy is created
(ie: "add backend"), EVENT_HDL_SUB_PROXY_DEL when backend is removed,
EVENT_HDL_SUB_PROXY_PUB when backend is published and
EVENT_HDL_SUB_PROXY_UNPUB when backend is unpublished.

All events provide event_hdl_cb_data_proxy struct, which contains info
about the proxy that triggered the event.