Add dedicated cluster and endpoint discovery services

[samuel-williams-shopify]

Summary

Add resource-specific Cluster Discovery Service (CDS) and Endpoint Discovery Service (EDS) implementations alongside the existing Aggregated Discovery Service.

This allows a local control plane to publish the cluster and endpoint resources it owns without claiming Envoy's single ADS stream. A separate control plane can continue to use ADS for listeners, routes, or other configuration.

Design

• Extract the state-of-the-world stream implementation shared by ADS, CDS, and EDS.
• Pin dedicated streams to their implied resource type.
• Accept an omitted type_url on CDS/EDS requests, as permitted for non-aggregated services.
• Reject a conflicting type_url rather than serving the wrong resource type.
• Ignore unrelated control-plane change notifications on dedicated streams.
• Keep delta discovery explicitly unimplemented.
• Allow Async::GRPC::XDS::Server to select the discovery services it registers; ADS remains the default.

Verification

• bundle exec bake test (79 examples)
• bundle exec rubocop
• bundle exec bake decode:index:coverage lib/async (118/118)