AIP#31 Event::UpdateTargeting (UPDATE_TARGETING)

[elpiel]

Sentry is still missing some impl details that were already made in the JS Sentry.

Since the introduction of https://github.com/AdExNetwork/aips/issues/31 there is a new Event type that was added called UPDATE_TARGETING.

TODO:

• Add the new Event type
• Update the channel
• Apply the correct access restrictions in sentry/src/access.rs
• Test additions in access.rs
• impl tests for the route
  Note: This is related to EventAggregator changes #319 since it's calling the EventAggregator