vici: Raise alert events from the bus alert listener hook

vici: Raise alert events from the bus alert listener hook

The bus alert infrastructure is currently exposed through the error-notify
plugin using a dedicated socket using a rather archaic message format.
Vici clients would need a dedicated socket connection just to receive such
alert messages, making their implementation more complex.

With vici, it is rather trivial to expose bus alerts through a dedicated
event message that vici clients may subscribe to. Add such an "alert"
event type to vici. Alert names are mapped to strings for simple consumption by
clients.

For now, the error-notify string message is omitted from events, as it mostly
contains static information without much value; instead add the IKE_SA details
for alerts associated to an IKE_SA. Other alert specific data may be added in
the future if needed; preferably using a structured format instead of the
arbitrary string messages used by error-notify. To allow future extensions,
wrap IKE_SA details under a dedicated "ike-sa" property.