Create gobject-introspection to make native plugins introspectable

There's a lot to this, but the documentation covers the consumer side. From
the GPlugin developer side, we basically created static library that manually
loads plugins and is setup in a way to be called via g-ir-scanner's --program
argument.

Testing Done:
Ran the new unit tests that test gir generation. Also installed to a local prefix and used that to compile a binary against `gplugin-introspection` and verified it displayed the proper output for `--help`.

Reviewed at https://reviews.imfreedom.org/r/2458/