Add ORCA protocol and load balancing support

[samuel-williams-shopify]

Summary

• add canonical ORCA load report and OpenRcaService definitions
• add Envoy 1.39 client-side weighted-round-robin policy definitions
• build typed ORCA load-balancing policies with an alternative reporting port
• publish endpoint hostnames through EDS for per-worker ORCA identity

Generated protobuf files remain excluded from documentation and coverage checks.

Verification

• bundle exec bake test
• bundle exec rubocop
• bundle exec bake decode:index:coverage lib/async