Adds a new document transformer that automatically extracts metadata for
a document based on an input schema. I also moved
`document_transformers.py` to `document_transformers/__init__.py` to
group it with this new transformer - it didn't seem to cause issues in
the notebook, but let me know if I've done something wrong there.
Also had a linter issue I couldn't figure out:
```
MacBook-Pro:langchain jacoblee$ make lint
poetry run mypy .
docs/dist/conf.py: error: Duplicate module named "conf" (also at "./docs/api_reference/conf.py")
docs/dist/conf.py: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#mapping-file-paths-to-modules for more info
docs/dist/conf.py: note: Common resolutions include: a) using `--exclude` to avoid checking one of them, b) adding `__init__.py` somewhere, c) using `--explicit-package-bases` or adjusting MYPYPATH
Found 1 error in 1 file (errors prevented further checking)
make: *** [lint] Error 2
```
@rlancemartin @baskaryan
---------
Co-authored-by: Bagatur <baskaryan@gmail.com>
