MINOR: server/event_hdl: prepare for server event data wrapper

MINOR: server/event_hdl: prepare for server event data wrapper

Adding the possibility to publish an event using a struct wrapper
around existing SERVER events to provide additional contextual info.

Using the specific struct wrapper is not required: it is supported
to cast event data as a regular server event data struct so
that we don't break the existing API.

However, casting event data with a more explicit data type allows
to fetch event-only relevant hints.