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/csv_loader.py

@@ -1,6 +1,7 @@
 import csv
 from io import TextIOWrapper
-from typing import Any, Dict, Iterator, List, Optional, Sequence
+from pathlib import Path
+from typing import Any, Dict, Iterator, List, Optional, Sequence, Union
 
 from langchain_core.documents import Document
 
@@ -35,7 +36,7 @@ class CSVLoader(BaseLoader):
 
     def __init__(
         self,
-        file_path: str,
+        file_path: Union[str, Path],
         source_column: Optional[str] = None,
         metadata_columns: Sequence[str] = (),
         csv_args: Optional[Dict] = None,
@@ -89,7 +90,7 @@ class CSVLoader(BaseLoader):
                 source = (
                     row[self.source_column]
                     if self.source_column is not None
-                    else self.file_path
+                    else str(self.file_path)
                 )
             except KeyError:
                 raise ValueError(