Add load-aware Envoy cluster example

[samuel-williams-shopify]

Summary

• configure the cluster example to discover its Envoy cluster dynamically through CDS and EDS
• enable out-of-band ORCA reports and client-side weighted round-robin balancing
• document load reporting, worker restarts, and the shared network topology
• improve the example image build layering

Verification

• bundle exec sus (110 tests, 476 assertions)
• targeted RuboCop checks
• Envoy configuration validation
• Docker Compose end-to-end requests through Envoy
• verified two active ORCA sessions, received reports, distinct worker weights, and endpoint/report replacement after killing a worker

Dependencies

Merge and release these first:

• Expose per-worker utilization snapshots async-service-supervisor#10
• Add ORCA protocol and load balancing support async-grpc-xds#3
• Add supervisor-driven ORCA load reporting async-service-supervisor-envoy#5