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/acreom.py

@@ -1,6 +1,6 @@
 import re
 from pathlib import Path
-from typing import Iterator
+from typing import Iterator, Union
 
 from langchain_core.documents import Document
 
@@ -14,7 +14,10 @@ class AcreomLoader(BaseLoader):
     """Regex to match front matter metadata in markdown files."""
 
     def __init__(
-        self, path: str, encoding: str = "UTF-8", collect_metadata: bool = True
+        self,
+        path: Union[str, Path],
+        encoding: str = "UTF-8",
+        collect_metadata: bool = True,
     ):
         """Initialize the loader."""
         self.file_path = path