### 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
ClusterState clusterState = clusterService.state();
            ClusterStateObserver observer = new ClusterStateObserver(clusterState, clusterService, null, logger, thread.getThreadContext());
            if (clusterState.nodes().getMasterNodeId() == null) {
                logger.debug("waiting to join the cluster. timeout [{}]", initialStateTimeout);
                final CountDownLatch latch = new CountDownLatch(1);
                observer.waitForNextChange(new ClusterStateObserver.Listener() {
                    @Override
                    public void onNewClusterState(ClusterState state) {
                        latch.countDown();
                    }
                    @Override
                    public void onClusterServiceClose() {
                        latch.countDown();
                    }
                    @Override
                    public void onTimeout(TimeValue timeout) {
                        logger.warn("timed out while waiting for initial discovery state - timeout: {}", initialStateTimeout);
                        latch.countDown();
                    }
                }, state -> state.nodes().getMasterNodeId() != null, initialStateTimeout);
                try {
                    latch.await();
                } catch (InterruptedException e) {
                    throw new ElasticsearchTimeoutException("Interrupted while waiting for initial discovery state");
                }
            }
        }
        injector.getInstance(HttpServerTransport.class).start();
        if (WRITE_PORTS_FILE_SETTING.get(settings())) {
            TransportService transport = injector.getInstance(TransportService.class);