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/html_bs.py

@@ -1,4 +1,5 @@
 import logging
+from pathlib import Path
 from typing import Dict, Iterator, Union
 
 from langchain_core.documents import Document
@@ -13,7 +14,7 @@ class BSHTMLLoader(BaseLoader):
 
     def __init__(
         self,
-        file_path: str,
+        file_path: Union[str, Path],
         open_encoding: Union[str, None] = None,
         bs_kwargs: Union[dict, None] = None,
         get_text_separator: str = "",
@@ -57,7 +58,7 @@ class BSHTMLLoader(BaseLoader):
             title = ""
 
         metadata: Dict[str, Union[str, None]] = {
-            "source": self.file_path,
+            "source": str(self.file_path),
             "title": title,
         }
         yield Document(page_content=text, metadata=metadata)