xds: Add LEDS list collection support (A95)

[shivaspeaks]

Implements part 2 of gRFC A95: support for fetching a locality's endpoints as a separate LbEndpointCollection (LEDS) list collection resource

[shivaspeaks]

Note for Reviewer

1. Map Values use StatusOr: The gRFC struct defines std::map<std::string, XdsLbEndpointCollectionResource>, but this PR models it as ImmutableMap<String, StatusOr<LbEndpointCollectionUpdate>>. This matches how XdsConfig handles all other resources (e.g., getClusters()). It ensures that one bad collection gracefully degrades only a single locality, rather than failing the entire cluster.

2. No separate resolution_note field: The gRFC specifies a resolution_note string, but today grpc-java folds resolution errors into the existing StatusOr<EdsUpdate> payload.

3. Missing or Invalid Collections ⇒ Locality Unreachable: The gRFC only explicitly defines that empty entries make a locality unreachable. I extended this behavior to missing or invalid collections. The observable state is identical, and it prevents a single missing LEDS resource from failing the cluster. (If all localities fail this way, CdsLoadBalancer2 still correctly reports "No usable endpoint from cluster").

4. Empty leds_collection_name: Not explicitly rejected during parsing; instead, it gracefully degrades into a never-arriving resource, rendering the locality unreachable.

5. NACK Blast Radius: A NACK'd collection fails only that specific collection. This aligns with standard XdsResourceType behavior, keeping the blast radius strictly scoped.

6. Resource Keys: shouldRetrieveResourceKeysForArgs() is set to true, mirroring EDS rather than RDS/LDS.

7. Updating EndpointConfig.equals() and hashCode() to include the new map is not just cosmetic. Without this, maybePublishConfig() would suppress updates that compare equal to the last one. A LEDS-only endpoint change (the entire point of this feature) would be silently swallowed because the underlying EDS resource wouldn't have changed. I added the verify_ledsCollectionUpdate_republishesConfig test specifically to pin this behavior.