Problem: unable to get the execution plan with select #807

This is problematic in tests and administrative functions, because we sometimes want just to extract something from the JSON output of the plan for testing purposes.

However, we don't have much choice but to use dblink and this would only work on objects already committed. Often not the case, especially in tests.

Proposed solution: implement explain function(s), returning json (and text?) of the plan

Should it go into omni_sql or some other extension? Do we need a new extension?