### 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
*/
    private final Logger logger = LogManager.getLogger(Node.class);
    private final DeprecationLogger deprecationLogger = DeprecationLogger.getLogger(Node.class);
    private final Injector injector;
    private final Environment environment;
    private final NodeEnvironment nodeEnvironment;
    private final PluginsService pluginsService;
    private final NodeClient client;
    private final Collection<LifecycleComponent> pluginLifecycleComponents;
    private final LocalNodeFactory localNodeFactory;
    private final NodeService nodeService;
    
    final NamedWriteableRegistry namedWriteableRegistry;
    final NamedXContentRegistry namedXContentRegistry;
    public Node(Environment environment) {
        this(environment, Collections.emptyList(), true);
    }
    /**
     * Constructs a node
     *
     * @param initialEnvironment         the initial environment for this node, which will be added to by plugins
     * @param classpathPlugins           the plugins to be loaded from the classpath
     * @param forbidPrivateIndexSettings whether or not private index settings are forbidden when creating an index; this is used in the
     *                                   test framework for tests that rely on being able to set private settings
     */
    protected Node(
        final Environment initialEnvironment,
        Collection<Class<? extends Plugin>> classpathPlugins,
        boolean forbidPrivateIndexSettings
    ) {