Use pathlib.Path in Directory layer in Python bindings if possible

Currently path handling done internally with tuples via `_to_unicode_path` function in `bindings/python/fdb/directory_impl.py`. Maybe it is better to do this via `pathlib.Path`. This came up in a discussion in #11418.

Start with bindings/python/fdb/directory_impl.py and inspect how _to_unicode_path represents and handles paths. Read the discussion in issue #11418 to understand the proposed pathlib.Path change; done means determining whether the replacement preserves the Directory layer's current path behavior and implementing it if the approach is suitable.

Written by the indexing model from the issue text.