fix: move read event from iterator to event.h

fix: move read event from iterator to event.h

because it is CTF-specific.

Need to move the format registry to libbabeltrace due to
cross-dependencies between libbabeltrace and formats.

Now, the format libs depend on libbabeltrace, never the opposite.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>