Add source metadata to OutlookMessageLoader (#11183)

Description: Add "source" metadata to OutlookMessageLoader

This pull request adds the "source" metadata to the OutlookMessageLoader
class in the load method. The "source" metadata is required when
indexing with RecordManager in order to sync the index documents with a
source.

Issue: None

Dependencies: None

Twitter handle: @ATelders

Co-authored-by: Arthur Telders <arthur.telders@roquette.com>

libs/langchain/langchain/document_loaders/email.py

@@ -107,6 +107,7 @@ def load(self) -> List[Document]:
             Document(
                 page_content=msg.body,
                 metadata={
+                    "source": self.file_path,
                     "subject": msg.subject,
                     "sender": msg.sender,
                     "date": msg.date,