### 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.indices.breaker.CircuitBreakerService;
import org.elasticsearch.indices.breaker.HierarchyCircuitBreakerService;
import org.elasticsearch.indices.breaker.NoneCircuitBreakerService;
import org.elasticsearch.indices.cluster.IndicesClusterStateService;
import org.elasticsearch.indices.recovery.PeerRecoverySourceService;
import org.elasticsearch.indices.recovery.PeerRecoveryTargetService;
import org.elasticsearch.indices.recovery.RecoverySettings;
import org.elasticsearch.indices.recovery.SnapshotFilesProvider;
import org.elasticsearch.indices.recovery.plan.RecoveryPlannerService;
import org.elasticsearch.indices.recovery.plan.ShardSnapshotsService;
import org.elasticsearch.indices.recovery.plan.SourceOnlyRecoveryPlannerService;
import org.elasticsearch.indices.store.IndicesStore;
import org.elasticsearch.ingest.IngestService;
import org.elasticsearch.monitor.MonitorService;
import org.elasticsearch.monitor.fs.FsHealthService;
import org.elasticsearch.monitor.jvm.JvmInfo;
import org.elasticsearch.persistent.PersistentTasksClusterService;
import org.elasticsearch.persistent.PersistentTasksExecutor;
import org.elasticsearch.persistent.PersistentTasksExecutorRegistry;
import org.elasticsearch.persistent.PersistentTasksService;
import org.elasticsearch.plugins.ActionPlugin;
import org.elasticsearch.plugins.AnalysisPlugin;
import org.elasticsearch.plugins.CircuitBreakerPlugin;
import org.elasticsearch.plugins.ClusterPlugin;
import org.elasticsearch.plugins.DiscoveryPlugin;
import org.elasticsearch.plugins.EnginePlugin;
import org.elasticsearch.plugins.HealthPlugin;
import org.elasticsearch.plugins.IndexStorePlugin;
import org.elasticsearch.plugins.IngestPlugin;
import org.elasticsearch.plugins.MapperPlugin;