### 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
}
    protected TransportService newTransportService(
        Settings settings,
        Transport transport,
        ThreadPool threadPool,
        TransportInterceptor interceptor,
        Function<BoundTransportAddress, DiscoveryNode> localNodeFactory,
        ClusterSettings clusterSettings,
        Set<String> taskHeaders
    ) {
        return new TransportService(settings, transport, threadPool, interceptor, localNodeFactory, clusterSettings, taskHeaders);
    }
    protected void processRecoverySettings(ClusterSettings clusterSettings, RecoverySettings recoverySettings) {
        
    }
    /**
     * The settings that are used by this node. Contains original settings as well as additional settings provided by plugins.
     */
    public Settings settings() {
        return this.environment.settings();
    }
    /**
     * A client that can be used to execute actions (operations) against the cluster.
     */
    public Client client() {
        return client;
    }
    /**
     * Returns the environment of the node
     */