Code enhancements for future use

[yhabteab]

This PR proposes the following improvements in preparation for #188.

• 9502b20 decouples contactChannels from the incident package, since we're going to use this recipient channels map elsewhere outside the incident package in the future and it seems reasonable to me to move it here to the rule package.
• d238b0b implements zapcore.ObjectMarshaler for the rule.Escalation and recipient.Key types and allows us to use zap.Inline(escalation) or zap.Object("escalation", escalation)wherever more logging context is needed without having to add all the individual fields ourselves each time.
• ce8732e moves the ErrSuperfluousStateChange error type to the event package in order to mitigate a cyclic import error with the upcoming changes to non-state event processing support. It kind of also fit into this package.
• 706ed38 drops incident#AddHistory() method as it was already quite annoying having to return types.Int every single time this method was used. This commit embeds the functionality for insert and fetch ID directly into the HistoryRow type and nowhere needs to be returned.
• 7ef9aa2 drops the create param from incident#ProcessEvent() and incident#GetCurrent(), since we can easily determine whether an incident is new based on its StartedAt timestamp, and thus, we don't need to pass this info around methods. (Added @sukhwinder33445 as a co-author as the original idea was from him, but I've implemented it in a slightly different way).

[yhabteab]

The output of zap.Object("escalation", escalation) looks as follows:

2024-05-17T10:05:49.375+0200    INFO    incident        Rule reached escalation {"object": "icinga-2!ping4", "incident": "#12", "rule": "default", "escalation": {"id": 1, "rule_id": 1, "name": "[S] Working Hours, [C] Yonas Habteab", "condition": "incident_severity>=ok"}}