[SPARK-2219][SQL] Added support for the "add jar" command

Adds logical and physical command classes for the "add jar" command.

Note that this PR conflicts with and should be merged after #2215.
Yea, I'll rebase this PR after #2215 is merged. `sideEffectResult` is required to return a `Seq[Row]` in #2215. So now, just let it be :)