Add support for ES Client Plugins

[anchialas]

Please add support for adding ES Client Plugins (like the org.elasticsearch.index.reindex.ReindexPlugin). Currently it's not possible to configure one or more plugins via EsConfiguration.

As a workaround I've to do something like:

ManagedEsClient prepareEsClient = new ManagedEsClient(configuration.getEsConfiguration());
TransportAddress[] addresses = TransportAddressHelper.fromHostAndPorts(configuration.getEsConfiguration().getServers());
// Rebuild Client with ReindexPlugin (unsupported in ManagedEsClient) 
ManagedEsClient elasticsearchClient = new ManagedEsClient(TransportClient.builder()
              .settings(prepareEsClient.getClient().settings())
              .addPlugin(ReindexPlugin.class)
              .build()
              .addTransportAddresses(addresses));