community[minor]: add document transformer for extracting links (#24186)

- **Description:** Add a DocumentTransformer for executing one or more
`LinkExtractor`s and adding the extracted links to each document.
- **Issue:** n/a
- **Depedencies:** none

---------

Co-authored-by: Eugene Yurtsev <eugene@langchain.dev>

libs/core/langchain_core/graph_vectorstores/links.py

@@ -12,7 +12,7 @@ class Link:
     """
 
     kind: str
-    """The kind of link. Allows different extractors to use the same tag name without 
+    """The kind of link. Allows different extractors to use the same tag name without
     creating collisions between extractors. For example “keyword” vs “url”."""
     direction: Literal["in", "out", "bidir"]
     """The direction of the link."""
@@ -66,3 +66,29 @@ def add_links(doc: Document, *links: Union[Link, Iterable[Link]]) -> None:
             links_in_metadata.extend(link)
         else:
             links_in_metadata.append(link)
+
+
+def copy_with_links(doc: Document, *links: Union[Link, Iterable[Link]]) -> Document:
+    """Return a document with the given links added.
+
+    Args:
+        doc: The document to add the links to.
+        *links: The links to add to the document.
+
+    Returns:
+        A document with a shallow-copy of the metadata with the links added.
+    """
+    new_links = set(get_links(doc))
+    for link in links:
+        if isinstance(link, Iterable):
+            new_links.update(link)
+        else:
+            new_links.add(link)
+
+    return Document(
+        page_content=doc.page_content,
+        metadata={
+            **doc.metadata,
+            METADATA_LINKS_KEY: list(new_links),
+        },
+    )