### 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
StopWatch stopWatch = new StopWatch("node_close");
        toClose.add(() -> stopWatch.start("node_service"));
        toClose.add(nodeService);
        toClose.add(() -> stopWatch.stop().start("http"));
        toClose.add(injector.getInstance(HttpServerTransport.class));
        toClose.add(() -> stopWatch.stop().start("snapshot_service"));
        toClose.add(injector.getInstance(SnapshotsService.class));
        toClose.add(injector.getInstance(SnapshotShardsService.class));
        toClose.add(injector.getInstance(RepositoriesService.class));
        toClose.add(() -> stopWatch.stop().start("client"));
        Releasables.close(injector.getInstance(Client.class));
        toClose.add(() -> stopWatch.stop().start("indices_cluster"));
        toClose.add(injector.getInstance(IndicesClusterStateService.class));
        toClose.add(() -> stopWatch.stop().start("indices"));
        toClose.add(injector.getInstance(IndicesService.class));
        
        toClose.add(injector.getInstance(IndicesStore.class));
        toClose.add(injector.getInstance(PeerRecoverySourceService.class));
        toClose.add(() -> stopWatch.stop().start("cluster"));
        toClose.add(injector.getInstance(ClusterService.class));
        toClose.add(() -> stopWatch.stop().start("node_connections_service"));
        toClose.add(injector.getInstance(NodeConnectionsService.class));
        toClose.add(() -> stopWatch.stop().start("cluster_coordinator"));
        toClose.add(injector.getInstance(Coordinator.class));
        toClose.add(() -> stopWatch.stop().start("monitor"));
        toClose.add(nodeService.getMonitorService());
        toClose.add(() -> stopWatch.stop().start("fsHealth"));
        toClose.add(injector.getInstance(FsHealthService.class));
        toClose.add(() -> stopWatch.stop().start("gateway"));
        toClose.add(injector.getInstance(GatewayService.class));