notifications: drop obsolete event fields & introduce new ones

[yhabteab]

Warning

This PR is not yet part of the main branch since it introduces a breaking change to the notifications package and all its users must be fixed first before merging this into the main branch (See the failing GHAs below).

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.