### 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.cluster.node.DiscoveryNode;
import org.elasticsearch.cluster.node.DiscoveryNodeRole;
import org.elasticsearch.cluster.routing.BatchedRerouteService;
import org.elasticsearch.cluster.routing.RerouteService;
import org.elasticsearch.cluster.routing.allocation.DiskThresholdMonitor;
import org.elasticsearch.cluster.service.ClusterService;
import org.elasticsearch.common.StopWatch;
import org.elasticsearch.common.breaker.CircuitBreaker;
import org.elasticsearch.common.component.Lifecycle;
import org.elasticsearch.common.component.LifecycleComponent;
import org.elasticsearch.common.inject.Injector;
import org.elasticsearch.common.inject.Key;
import org.elasticsearch.common.inject.ModulesBuilder;
import org.elasticsearch.common.io.stream.NamedWriteableRegistry;
import org.elasticsearch.common.logging.DeprecationCategory;
import org.elasticsearch.common.logging.DeprecationLogger;
import org.elasticsearch.common.logging.HeaderWarning;
import org.elasticsearch.common.logging.NodeAndClusterIdStateListener;
import org.elasticsearch.common.network.NetworkAddress;
import org.elasticsearch.common.network.NetworkModule;
import org.elasticsearch.common.network.NetworkService;
import org.elasticsearch.common.settings.ClusterSettings;
import org.elasticsearch.common.settings.ConsistentSettingsService;
import org.elasticsearch.common.settings.Setting;
import org.elasticsearch.common.settings.Setting.Property;
import org.elasticsearch.common.settings.SettingUpgrader;
import org.elasticsearch.common.settings.Settings;
import org.elasticsearch.common.settings.SettingsModule;
import org.elasticsearch.common.transport.BoundTransportAddress;
import org.elasticsearch.common.transport.TransportAddress;