ENH: Support native Transport client (as alternative to http)

[rbygrave]

Currently the communication to ElasticSearch is via http. It would be good to add support for using the native transport client (as embedded client or full embedded server).

[icode]

perfect, waiting it! Early hopes for support

[icode]

Nexus oss 3.x is use es embedded, it easy use, and use for file indexes. Please support

[icode]

most do not use the database when using local or embedded es, such as nexus oss 3.x https://github.com/sonatype/nexus-public/blob/master/components/nexus-repository/src/main/java/org/sonatype/nexus/repository/search/SearchServiceImpl.java

And do not use the es cluster, there is performance overhead when using http access