common: Python-related bits in bs_plugin_common.h
common: Python-related bits in bs_plugin_common.h

- plugin_descriptor initialized as static variable istead of allocation
  with new as earlier (change macro);
- bs_init_py_subsystem() now accept pybind11::module reference
  corresponding to plugin's Python namespace.

uentity/blue-sky-re,uentity/blue-sky-re,uentity/blue-sky-re