Add (re-implement) support for Solr's HDFSDirectory

[mocobeta]

I have dropped this feature when porting Thinlet UI to JavaFX (later, replaced by Swing.)

I would like to re-think this feature. (Further more, it might be much better if Luke supports arbitrary non-FS based directory implementations, is it possible?)

To implement this in a proper way, some research effort is needed because of my lack of knowledge about the directory class. I myself do not need the feature for now, so the priority is up to users...
I am not sure that how many Thinlet Luke users use this feature, comments or votes (+1) are helpful to estimate the importance of the issue. (Of course PRs are most welcome.)

[mocobeta]

For contributors: Now Luke is a part of Apache Lucene. Please create an issue (if there is not exist) to Lucene Jira if you want to help with this.

[mocobeta]

I moved this to the Lucene JIRA: https://issues.apache.org/jira/browse/LUCENE-8771