operator_registry: `meta` field to event
operator_registry: `meta` field to event

New optional `meta` field is introduced to the event structure in order
to serve future purposes on so called `meta-requests` serving
information about the OperationRegistry and the registered operations.

Signed-off-by: Gergő Nagy <1a73af9e7ae00182733b2292511b814be66f065f@gergonagy.com>

ngergo/ratatoskr