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/text.py

@@ -1,5 +1,6 @@
 import logging
-from typing import Iterator, Optional
+from pathlib import Path
+from typing import Iterator, Optional, Union
 
 from langchain_core.documents import Document
 
@@ -25,7 +26,7 @@ class TextLoader(BaseLoader):
 
     def __init__(
         self,
-        file_path: str,
+        file_path: Union[str, Path],
         encoding: Optional[str] = None,
         autodetect_encoding: bool = False,
     ):
@@ -56,5 +57,5 @@ class TextLoader(BaseLoader):
         except Exception as e:
             raise RuntimeError(f"Error loading {self.file_path}") from e
 
-        metadata = {"source": self.file_path}
+        metadata = {"source": str(self.file_path)}
         yield Document(page_content=text, metadata=metadata)