feat: add federated event Source/Resource/Origin support

[savme]

Builds on the schema from #240 to populate source. An event carrying source-* annotations (stamped by an edge exporter) now gets those values copied onto the Activity's source field.

Test plan

• go test
• Table-driven coverage for source population, related-based resource resolution, and origin ID qualification, including the same-UID-different-cluster case

Related to datum-cloud/compute#100

Stack:

• fix: shared processor event extractors #239
• feat: add federated events schema #240
• feat: add federated event Source/Resource/Origin support #245 👈
• feat: add federated events exporter #246
• feat: qualify dedup IDs and bound the exporter queue #248
• feat: federated events transport #250