feat: add federated events RFC

[savme]

Extends Activity's event pipeline to watch and attribute events from Compute's edge clusters, not just the management plane - making instance-level activity visible, filterable by location, and traceable back to the workload that caused it.

Key things:

• Evolve the existing event-exporter and deploy one per edge cluster
• Add a typed Source field (plane, cluster, region, city) to the Activity API
• Change event identity to include plane and cluster, since raw Kubernetes UIDs can collide across independently-created clusters
• Recover tenant scope from namespace labels Karmada already manages
• Correlate edge activities back to their originating workload through a resource link
• Authenticate each edge exporter to NATS with an nkey

Related to datum-cloud/compute#100

[scotwells]

Overall looks good. Let's use locations.miloapis.com/location to line up with the new milo-os/locations service we're introducing. This label should contain the location name (e.g. us-central-1).