Put Python plugin support in a separate shared object

Put Python plugin support in a separate shared object

This is to make the work of packagers easier for libbabeltrace not
to depend on libpython, even if you need Python plugin support.

With this patch, python.c's constructor tries to open the new Python
plugin provider shared object. If it fails, Python plugin support is
disabled, but shared object plugins can still be loaded.

The new BUILT_IN_PYTHON_PLUGIN_SUPPORT configure variable, if set to 1,
makes the build embed the Python plugin provider into libbabeltrace
(like before this patch).

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>