### 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
final List<Closeable> resourcesToClose = new ArrayList<>(); 
        boolean success = false;
        try {
            
            DeprecationLogger.initialize(initialEnvironment.settings());
            Settings tmpSettings = Settings.builder()
                .put(initialEnvironment.settings())
                .put(Client.CLIENT_TYPE_SETTING_S.getKey(), CLIENT_TYPE)
                .build();
            final JvmInfo jvmInfo = JvmInfo.jvmInfo();
            logger.info(
                "version[{}], pid[{}], build[{}/{}/{}/{}], OS[{}/{}/{}], JVM[{}/{}/{}/{}]",
                Build.CURRENT.qualifiedVersion(),
                jvmInfo.pid(),
                Build.CURRENT.flavor().displayName(),
                Build.CURRENT.type().displayName(),
                Build.CURRENT.hash(),
                Build.CURRENT.date(),
                Constants.OS_NAME,
                Constants.OS_VERSION,
                Constants.OS_ARCH,
                Constants.JVM_VENDOR,
                Constants.JVM_NAME,
                Constants.JAVA_VERSION,
                Constants.JVM_VERSION
            );
            if (jvmInfo.getBundledJdk()) {
                logger.info("JVM home [{}], using bundled JDK [{}]", System.getProperty("java.home"), jvmInfo.getUsingBundledJdk());
            } else {
                logger.info("JVM home [{}]", System.getProperty("java.home"));