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/telegram.py

@@ -25,7 +25,7 @@ def concatenate_rows(row: dict) -> str:
 class TelegramChatFileLoader(BaseLoader):
     """Load from `Telegram chat` dump."""
 
-    def __init__(self, path: str):
+    def __init__(self, path: Union[str, Path]):
         """Initialize with a path."""
         self.file_path = path