[FEA] spark 2.x explain api integrated into sql-plugin

[tgravescs]

Is your feature request related to a problem? Please describe.
The current proposal for the Spark 2.x explain api is to put it in a separate module. This is due to timing and not finding a good way to separate tagging and conversion logic in sql-plugin. In a follow on we want to integration the code so we don't have 2 copies of a bunch of stuff.

We also want to add automated tests where possible.