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/rst.py

@@ -1,5 +1,6 @@
 """Loads RST files."""
-from typing import Any, List
+from pathlib import Path
+from typing import Any, List, Union
 
 from langchain_community.document_loaders.unstructured import (
     UnstructuredFileLoader,
@@ -32,7 +33,10 @@ class UnstructuredRSTLoader(UnstructuredFileLoader):
     """
 
     def __init__(
-        self, file_path: str, mode: str = "single", **unstructured_kwargs: Any
+        self,
+        file_path: Union[str, Path],
+        mode: str = "single",
+        **unstructured_kwargs: Any,
     ):
         """
         Initialize with a file path.