### Description

Implement health indicator for shard allocation.
This indicator should have same or similar behavior to existing health endpoint.

Aspects to be considered:
* nodes scheduled for reboot should not cause cluster to go yellow (but could cause it go red)
* huge number of unallocated shards should not cause response bloat.
* consider showing allocation explain if possible

Rel: TBD
server/src/main/java/org/elasticsearch/node/Node.java
import org.elasticsearch.action.search.SearchExecutionStatsCollector;
import org.elasticsearch.action.search.SearchPhaseController;
import org.elasticsearch.action.search.SearchTransportService;
import org.elasticsearch.action.support.StatsRequestLimiter;
import org.elasticsearch.action.support.TransportAction;
import org.elasticsearch.action.update.UpdateHelper;
import org.elasticsearch.bootstrap.BootstrapCheck;
import org.elasticsearch.bootstrap.BootstrapContext;
import org.elasticsearch.client.internal.Client;
import org.elasticsearch.client.internal.node.NodeClient;
import org.elasticsearch.cluster.ClusterInfoService;
import org.elasticsearch.cluster.ClusterModule;
import org.elasticsearch.cluster.ClusterName;
import org.elasticsearch.cluster.ClusterState;
import org.elasticsearch.cluster.ClusterStateObserver;
import org.elasticsearch.cluster.InternalClusterInfoService;
import org.elasticsearch.cluster.NodeConnectionsService;
import org.elasticsearch.cluster.action.index.MappingUpdatedAction;
import org.elasticsearch.cluster.coordination.Coordinator;
import org.elasticsearch.cluster.coordination.InstanceHasMasterHealthIndicatorService;
import org.elasticsearch.cluster.desirednodes.DesiredNodesSettingsValidator;
import org.elasticsearch.cluster.metadata.IndexMetadataVerifier;
import org.elasticsearch.cluster.metadata.IndexTemplateMetadata;
import org.elasticsearch.cluster.metadata.Metadata;
import org.elasticsearch.cluster.metadata.MetadataCreateDataStreamService;
import org.elasticsearch.cluster.metadata.MetadataCreateIndexService;
import org.elasticsearch.cluster.metadata.MetadataDataStreamsService;
import org.elasticsearch.cluster.metadata.MetadataUpdateSettingsService;
import org.elasticsearch.cluster.metadata.SystemIndexMetadataUpgradeService;
import org.elasticsearch.cluster.metadata.TemplateUpgradeService;