Add `src/py-common`, containing bt_py_common_format_exception() for now

This new convenience library is meant to contain functions used by
Python bindings and the Python plugin provider.

As of this patch, it only offers bt_py_common_format_exception(), which
is similar to what bt2_py_loge_exception() used to do in
`native_bt_component_class.i`, except that it returns the formatted
exception string instead of logging it directly. Now
bt2_py_loge_exception() uses bt_py_common_format_exception().

bt_py_common_format_exception() will be needed in a future patch to set
the message of an error cause when appending from the Python plugin
provider.

No functional change intended.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Iaa9dc081e7d8a3d6c85420fcbbfc045a9d79bf27
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1556
Tested-by: jenkins <jenkins@lttng.org>