Add support for additional, optional CLI arg to "salt-run thin.generate"...

... (which gets passed through to ssh_py_shim) to speciy comma separated list of *native* python module names to be packaged up in the thin tar (ie: single-file python modules implemented as .so files). This type of module does not play well "normally" using the extra_mods functionality of thin.generate, and implicit defaults do not work for native extensions where the master/minion are in a heterogenous environment, ie: non-matching architectures.

This way, .so modules (native extensions) can be packaged into a thin tar even in the presence of a heterogeneous environment, assuming the master has available the correct binary .so module for the target minion architecture.

Addresses/solves/fixes issue #13769