Investigate the future of the Pig action

Pig is one of the oldest actions in Oozie, it was part of the original open source release.

Right now one important Oozie task ( OOZIE-2973 - Make sure Oozie works with Hadoop 3 ) is blocked by a PIG issue ( PIG-5253 ) for a very long time and I don't really think this issue will be solved soon:

• The latest Pig version (0.17) was released almost 2 years ago.
• There are only 6 commits in the project in 2019.

I think sooner or later we have to choose if we want to support Hadoop 3 or Pig, and Hadoop3 is much more important for Oozie. I think we may consider dropping Pig support. Of course such a big change should only targeted to 6.0, but we could already make it deprecated in the next minor (5.2) version.