xds: support LRS for logical DNS clusters

[dfawley]

It appears we don't ever set the necessary LRS fields in order for LRS to work in logical DNS clusters; only in EDS:

grpc-go/xds/internal/balancer/cdsbalancer/cdsbalancer.go

Lines 610 to 638 in e14d583

	case xdsresource.ClusterTypeEDS:
	dm = clusterresolver.DiscoveryMechanism{
	Type: clusterresolver.DiscoveryMechanismTypeEDS,
	Cluster: cluster.ClusterName,
	EDSServiceName: cluster.EDSServiceName,
	MaxConcurrentRequests: cluster.MaxRequests,
	}
	if cluster.LRSServerConfig == xdsresource.ClusterLRSServerSelf {
	bootstrapConfig := b.xdsClient.BootstrapConfig()
	parsedName := xdsresource.ParseName(cluster.ClusterName)
	if parsedName.Scheme == xdsresource.FederationScheme {
	// Is a federation resource name, find the corresponding
	// authority server config.
	if cfg, ok := bootstrapConfig.Authorities[parsedName.Authority]; ok {
	dm.LoadReportingServer = cfg.XDSServer
	}
	} else {
	// Not a federation resource name, use the default
	// authority.
	dm.LoadReportingServer = bootstrapConfig.XDSServer
	}
	}
	case xdsresource.ClusterTypeLogicalDNS:
	dm = clusterresolver.DiscoveryMechanism{
	Type: clusterresolver.DiscoveryMechanismTypeLogicalDNS,
	Cluster: cluster.ClusterName,
	DNSHostname: cluster.DNSHostName,
	}
	}

We should make sure it works for logical DNS clusters, too, even though we don't currently configure this today.