DDLSHELL-45. Support extended environment attrs for modules.

Adds EnvironmentPlugin SPI.
Modifies Environment and DDLCommand to track per-module state.
Adds unit tests.
DDLSHELL-43. Add USE JAR INFILE... command.

The Environment retains a list of 'libJars' that specify jar resources that
should be made available to subprocesses.
intermediate: DDLSHELL-44. Support Java subprocess execution.

Adds the ForkJvm trait to assist DDLCommand implementations in
launching Java subprocesses.

Adds the 'jvm' plugin module with the FORK JVM 'com.example.MyMain' command.

THIS NEEDS TESTS

THIS NEEDS TESTS