### 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
}
            if (Environment.dataPathUsesList(tmpSettings)) {
                
                deprecationLogger.warn(
                    DeprecationCategory.SETTINGS,
                    "multiple-data-paths-list",
                    "Configuring [path.data] with a list is deprecated. Instead specify as a string value."
                );
            }
            if (logger.isDebugEnabled()) {
                logger.debug(
                    "using config [{}], data [{}], logs [{}], plugins [{}]",
                    initialEnvironment.configFile(),
                    Arrays.toString(initialEnvironment.dataFiles()),
                    initialEnvironment.logsFile(),
                    initialEnvironment.pluginsFile()
                );
            }
            this.pluginsService = new PluginsService(
                tmpSettings,
                initialEnvironment.configFile(),
                initialEnvironment.modulesFile(),
                initialEnvironment.pluginsFile(),
                classpathPlugins
            );
            final Settings settings = pluginsService.updatedSettings();
            /*
             * Create the environment based on the finalized view of the settings. This is to ensure that components get the same setting
             * values, no matter they ask for them from.
             */