Implement A61 update to remove special case for LOGICAL_DNS clusters

[arjan-bal]

An update was made to A61 in grpc/proposal#477 to remove the need for hardcoding pick_first as the child for the priority balancer for logical DNS clusters. The code for this is here:

grpc-go/xds/internal/balancer/clusterresolver/configbuilder.go

Lines 141 to 159 in 6dfe07c

	func buildClusterImplConfigForDNS(g *nameGenerator, endpoints []resolver.Endpoint, mechanism DiscoveryMechanism) (string, *clusterimpl.LBConfig, []resolver.Endpoint) {
	// Endpoint picking policy for DNS is hardcoded to pick_first.
	const childPolicy = "pick_first"
	retEndpoints := make([]resolver.Endpoint, len(endpoints))
	pName := fmt.Sprintf("priority-%v", g.prefix)
	for i, e := range endpoints {
	retEndpoints[i] = hierarchy.SetInEndpoint(e, []string{pName})
	// Copy the nested address field as slice fields are shared by the
	// iteration variable and the original slice.
	retEndpoints[i].Addresses = append([]resolver.Address{}, e.Addresses...)
	}
	return pName, &clusterimpl.LBConfig{
	Cluster: mechanism.Cluster,
	TelemetryLabels: mechanism.TelemetryLabels,
	ChildPolicy: &internalserviceconfig.BalancerConfig{Name: childPolicy},
	MaxConcurrentRequests: mechanism.MaxConcurrentRequests,
	LoadReportingServer: mechanism.LoadReportingServer,
	}, retEndpoints
	}

We should update the clusterresolver balancer accordingly.