notifications: drop obsolete event fields & introduce new ones

[yhabteab]

This PR drops the obsolete extra_tags, rule_ids, and rules_version fields from the notification Event struct as they are no longer used by Icinga Notifications since Icinga/icinga-notifications#418. It also introduces two new fields complete_relations and relations to the Event struct, which are going to be used by the new Notifications rule evaluation engine to determine which rules match a given event. Additionally, it adds a new HTTP header X-Icinga-Reject-If-Relations-Incomplete to be used by the Notifications API to indicate whether the client supports attribute negotiation. The name of the header is not final and is open for discussion.

[oxzi]

While I am generally okay with this change, the failing compatibility CI jobs underline a certain question: Do we want to merge this directly into main and temporarily breaking compatibility with the downstream consumers or do we want to work with a feature branch to merge things into before merging it all back to main?

[yhabteab]

Just in case we need to release IGL in the meantime without such changes, we can also use a separate branch till all the consumers are adapted to these changes as well.

[yhabteab]

I've changed the base branch now.

Review dismissed with https://github.com/Icinga/icinga-go-library/compare/1be9a41e15e80fe65e811c8a1f17a344010a4522..5024937aed30834cdfe13f9cd8340b6db68077e6

[yhabteab]

After a short discussion with @nilmerg offline, we agreed to rename the HTTP header to X-Icinga-Reject-If-Relations-Incomplete to reflect its usage correctly, so I've updated the PR accordingly.