core[minor]: Add Graph Store component (#23092)

This PR introduces a GraphStore component. GraphStore extends
VectorStore with the concept of links between documents based on
document metadata. This allows linking documents based on a variety of
techniques, including common keywords, explicit links in the content,
and other patterns.

This works with existing Documents, so it’s easy to extend existing
VectorStores to be used as GraphStores. The interface can be implemented
for any Vector Store technology that supports metadata, not only graph
DBs.

When retrieving documents for a given query, the first level of search
is done using classical similarity search. Next, links may be followed
using various traversal strategies to get additional documents. This
allows documents to be retrieved that aren’t directly similar to the
query but contain relevant information.

2 retrieving methods are added to the VectorStore ones : 
* traversal_search which gets all linked documents up to a certain depth
* mmr_traversal_search which selects linked documents using an MMR
algorithm to have more diverse results.

If a depth of retrieval of 0 is used, GraphStore is effectively a
VectorStore. It enables an easy transition from a simple VectorStore to
GraphStore by adding links between documents as a second step.

An implementation for Apache Cassandra is also proposed.

See
https://github.com/datastax/ragstack-ai/blob/main/libs/knowledge-store/notebooks/astra_support.ipynb
for a notebook explaining how to use GraphStore and that shows that it
can answer correctly to questions that a simple VectorStore cannot.

**Twitter handle:** _cbornet

libs/core/tests/unit_tests/test_graph_vectorstores.py

@@ -0,0 +1,59 @@
+import pytest
+
+from langchain_core.documents import Document
+from langchain_core.graph_vectorstores.base import (
+    Node,
+    _documents_to_nodes,
+    _texts_to_nodes,
+)
+from langchain_core.graph_vectorstores.links import Link
+
+
+def test_texts_to_nodes() -> None:
+    assert list(_texts_to_nodes(["a", "b"], [{"a": "b"}, {"c": "d"}], ["a", "b"])) == [
+        Node(id="a", metadata={"a": "b"}, text="a"),
+        Node(id="b", metadata={"c": "d"}, text="b"),
+    ]
+    assert list(_texts_to_nodes(["a", "b"], None, ["a", "b"])) == [
+        Node(id="a", metadata={}, text="a"),
+        Node(id="b", metadata={}, text="b"),
+    ]
+    assert list(_texts_to_nodes(["a", "b"], [{"a": "b"}, {"c": "d"}], None)) == [
+        Node(metadata={"a": "b"}, text="a"),
+        Node(metadata={"c": "d"}, text="b"),
+    ]
+    assert list(
+        _texts_to_nodes(
+            ["a"],
+            [{"links": {Link.incoming(kind="hyperlink", tag="http://b")}}],
+            None,
+        )
+    ) == [Node(links=[Link.incoming(kind="hyperlink", tag="http://b")], text="a")]
+    with pytest.raises(ValueError):
+        list(_texts_to_nodes(["a", "b"], None, ["a"]))
+    with pytest.raises(ValueError):
+        list(_texts_to_nodes(["a", "b"], [{"a": "b"}], None))
+    with pytest.raises(ValueError):
+        list(_texts_to_nodes(["a"], [{"a": "b"}, {"c": "d"}], None))
+    with pytest.raises(ValueError):
+        list(_texts_to_nodes(["a"], None, ["a", "b"]))
+
+
+def test_documents_to_nodes() -> None:
+    documents = [
+        Document(
+            id="a",
+            page_content="some text a",
+            metadata={"links": [Link.incoming(kind="hyperlink", tag="http://b")]},
+        ),
+        Document(id="b", page_content="some text b", metadata={"c": "d"}),
+    ]
+    assert list(_documents_to_nodes(documents)) == [
+        Node(
+            id="a",
+            metadata={},
+            links=[Link.incoming(kind="hyperlink", tag="http://b")],
+            text="some text a",
+        ),
+        Node(id="b", metadata={"c": "d"}, text="some text b"),
+    ]