### 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
}
    /**
     * Creates a new {@link BigArrays} instance used for this node.
     * This method can be overwritten by subclasses to change their {@link BigArrays} implementation for instance for testing
     */
    BigArrays createBigArrays(PageCacheRecycler pageCacheRecycler, CircuitBreakerService circuitBreakerService) {
        return new BigArrays(pageCacheRecycler, circuitBreakerService, CircuitBreaker.REQUEST);
    }
    /**
     * Creates a new {@link BigArrays} instance used for this node.
     * This method can be overwritten by subclasses to change their {@link BigArrays} implementation for instance for testing
     */
    PageCacheRecycler createPageCacheRecycler(Settings settings) {
        return new PageCacheRecycler(settings);
    }
    /**
     * Creates a new the SearchService. This method can be overwritten by tests to inject mock implementations.
     */
    protected SearchService newSearchService(
        ClusterService clusterService,
        IndicesService indicesService,
        ThreadPool threadPool,
        ScriptService scriptService,
        BigArrays bigArrays,
        FetchPhase fetchPhase,
        ResponseCollectorService responseCollectorService,
        CircuitBreakerService circuitBreakerService,
        ExecutorSelector executorSelector
    ) {
        return new SearchService(