### 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.plugins.MetadataUpgrader;
import org.elasticsearch.plugins.NetworkPlugin;
import org.elasticsearch.plugins.PersistentTaskPlugin;
import org.elasticsearch.plugins.Plugin;
import org.elasticsearch.plugins.PluginsService;
import org.elasticsearch.plugins.RecoveryPlannerPlugin;
import org.elasticsearch.plugins.RepositoryPlugin;
import org.elasticsearch.plugins.ScriptPlugin;
import org.elasticsearch.plugins.SearchPlugin;
import org.elasticsearch.plugins.ShutdownAwarePlugin;
import org.elasticsearch.plugins.SystemIndexPlugin;
import org.elasticsearch.repositories.RepositoriesModule;
import org.elasticsearch.repositories.RepositoriesService;
import org.elasticsearch.rest.RestController;
import org.elasticsearch.script.ScriptContext;
import org.elasticsearch.script.ScriptEngine;
import org.elasticsearch.script.ScriptModule;
import org.elasticsearch.script.ScriptService;
import org.elasticsearch.search.SearchModule;
import org.elasticsearch.search.SearchService;
import org.elasticsearch.search.SearchUtils;
import org.elasticsearch.search.aggregations.support.AggregationUsageService;
import org.elasticsearch.search.fetch.FetchPhase;
import org.elasticsearch.shutdown.PluginShutdownService;
import org.elasticsearch.snapshots.InternalSnapshotsInfoService;
import org.elasticsearch.snapshots.RepositoryIntegrityHealthIndicatorService;
import org.elasticsearch.snapshots.RestoreService;
import org.elasticsearch.snapshots.SnapshotShardsService;
import org.elasticsearch.snapshots.SnapshotsInfoService;
import org.elasticsearch.snapshots.SnapshotsService;