feat(graph): Flexible Schema Extension to SpannerGraphStore (#125)

* Extension to SpannerGraphStore

- Allow a dynamic schema for nodes and edges:
  - nodes are stored in one table with dynamic label and properties (JSON)
- Allow more than one (src.node.id, target.node.id, edge.type) by
  introducing a uuid as part of the key;
- Prefix the table names with graph names to avoid conflicting.

* Fix linters

* Update notebook for illustrate flexible schema

* Remove the uuid key

* Resolve comments

* Avoid unnecessary overwrites based on the specified properties

Previously, we override all columns by schema;
with this change, we selectively write specified columns.

Note:
- this won't change the behavior if you always do a full blind write;

* Address comments

* Minor updates to the interface

* Add agent info for usage tracking

* Update src/langchain_google_spanner/graph_store.py

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Co-authored-by: Averi Kitsch <akitsch@google.com>

Author: mtyin