Sbromberger/add backend path

[sbromberger]

import clippy
from clippy.backends import fs

# Scan this directory and add its generated classes to clippy.
fs.add_backend_path("/workspaces/clippy/cpp/build/examples")

# Classes discovered in that directory are exposed through clippy.
bag = clippy.ExampleBag()
bag.insert(42)

print(bag.size())  # 1