Allow explicit setting of extensions of hadoop tasks

[navis]

Cause hadoop tasks exploits single flat classloader, it's not possible to use competitive modules concurrently, like lucene7 and lucene8 extension.

it's just a temporary measure.

{
   "type":"index_hadoop",
   "extensions": [ "druid-lucene8-extensions"],
   "spec":{
      "dataSchema":{
         "dataSource":"lineitem",