feat(graph): add Spanner Graph support to LangChain GraphStore interface (googleapis#104)

* feat(graph): add Spanner Graph support to LangChain GraphStore interface

1) Add SpannerGraphStore implementation of GraphStore;
2) Add an integration test;
3) Add a notebook to demonstrate the usage;
4) Misc updates: requirements, __init__

* Allow more flexible input that can generated by LLMs

1) same edge label between different types of nodes

E.g.

Company FOCUS_ON Product
Company FOCUS_ON Technology

2) the same doc contains multiple nodes/edges with the same key fields.

E.g.

Merge
  Company(id='Google', properties={'p1':'a1', 'p3':'a3'}),
  Company(id='Google', properties={'p2':'a2', 'p3':'a4'})
into
  Company(id='Google', properties={'p1':'a1', 'p2':'a2', 'p3':'a4'})

This is due to Spanner DML doesn't support updating the same row twice
in the same DML. We could also separate each node/edge into a separate
DML which can be very slow.

* style: formatting

* style: formatting

* Address various review feedbacks

1) Fix style of python notebook;
2) GraphStore:
   - improve a corner case by avoiding full set iteration;
   - add some null checking;
   - move utilities around.

* Format the notebook

* Fix linter error

* Fix type annotation

* Use batch table insertion API instead of transaction DML API

* Minor fixes & cleanups: colab, requirements, tests, schema corner cases

---------

Co-authored-by: Gaurav Purohit <[email protected]>
Co-authored-by: gauravpurohit <[email protected]>

Author: mtyin

README.rst

@@ -131,6 +131,27 @@ See the full `Chat Message History`_ tutorial.
 
 .. _`Chat Message History`: https://github.com/googleapis/langchain-google-spanner-python/blob/main/docs/chat_message_history.ipynb
 
+Spanner Graph Store Usage
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Use ``SpannerGraphStore`` to store nodes and edges extracted from documents.
+
+.. code:: python
+
+   from langchain_google_spanner import SpannerGraphStore
+
+
+    graph = SpannerGraphStore(
+        instance_id="my-instance",
+        database_id="my-database",
+        graph_name="my_graph",
+    )
+
+See the full `Spanner Graph Store`_ tutorial.
+
+.. _`Spanner Graph Store`: https://github.com/googleapis/langchain-google-spanner-python/blob/main/docs/graph_store.ipynb
+
+
 Contributions
 ~~~~~~~~~~~~~