community: better support of pathlib paths in document loaders (#18396)

So this arose from the
https://github.com/langchain-ai/langchain/pull/18397 problem of document
loaders not supporting `pathlib.Path`.

This pull request provides more uniform support for Path as an argument.
The core ideas for this upgrade: 
- if there is a local file path used as an argument, it should be
supported as `pathlib.Path`
- if there are some external calls that may or may not support Pathlib,
the argument is immidiately converted to `str`
- if there `self.file_path` is used in a way that it allows for it to
stay pathlib without conversion, is is only converted for the metadata.

Twitter handle: https://twitter.com/mwmajewsk

libs/community/langchain_community/document_loaders/roam.py

@@ -1,5 +1,5 @@
 from pathlib import Path
-from typing import List
+from typing import List, Union
 
 from langchain_core.documents import Document
 
@@ -9,7 +9,7 @@ from langchain_community.document_loaders.base import BaseLoader
 class RoamLoader(BaseLoader):
     """Load `Roam` files from a directory."""
 
-    def __init__(self, path: str):
+    def __init__(self, path: Union[str, Path]):
         """Initialize with a path."""
         self.file_path = path