Watch and sync changes to related resources

[iakmc]

Summary

Previously, changes to related resources with origin: kcp (e.g. a
ConfigMap created by a user in a kcp workspace) were ignored — only
changes to the primary object triggered reconciliation.

This adds:

• RelatedObjectIndex: an in-memory reverse index mapping each kcp-side
  related object (cluster, group, resource, namespace, name) back to
  its owning primary object. Uses a struct as the map key instead of
  string concatenation.
• Index population: during reconciliation of the primary object, all
  resolved origin: kcp related objects are registered in the index.
• MultiClusterWatch per related resource type: when a related object
  changes in kcp, the watch handler looks it up in the index and enqueues
  the owning primary object for reconciliation, which then syncs the
  updated related object to the service cluster.

Note: the index is empty after an agent restart. It is repopulated
naturally as primary objects are reconciled.

What Type of PR Is This?

/kind feature

Related Issue(s)

Fixes #118

Release Notes

NONE

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[xrstf]

I think this is the completely wrong approach to solving this. This method has no way of handling label-selector-based related resources, for which the agent cannot possibly know that an objects belongs to the primary object. That's why in the original ticket I suggested that we need extra configuration, to allow the agent to inverse the relationship between primary and related object (i.e. so that it can tell, just from looking at a Secret, which primary object that Secret belongs to).

This index would just grow and grow and grow over time and not even notice anything that's not an immediate reference.