SpannerGraphVectorContextRetriever doesn't support Approximate Nearest Neighbor search

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#### Environment details

- OS type and version: MacOS Darwin Kernel Version 23.6.0 arm64
- Python version: 3.12.9
- pip version: pip 25.0.1
- `langchain-google-spanner` version: 0.8.1

#### Steps to reproduce

I'm unable to use APPROX_DOT_PRODUCT metric for vector search in the `SpannerGraphVectorContextRetriever`. Upon checking the implementation, I found that the `__get_distance_function` only supports COSINE and EUCLIDEAN distance.
```python
def __get_distance_function(
self, distance_strategy=DistanceStrategy.EUCLIDEAN
) -> str:
"""Gets the vector distance function."""
if distance_strategy == DistanceStrategy.COSINE:
return "COSINE_DISTANCE"

return "EUCLIDEAN_DISTANCE"
```

Here is an example Spanner Graph query I'm trying to create.
```sql
GRAPH TestGraph
MATCH (node:Node1)
WHERE node.embedding is not NULL
ORDER BY APPROX_DOT_PRODUCT(node.embedding, ARRAY[
-0.036149498,
0.07207133,
-0.020961108,
-0.005176733,
0.047590617,
0.006568967,
-0.047031555,
0.04741984
],
OPTIONS => JSON '{"num_leaves_to_search": 5}') DESC
LIMIT 5
RETURN node.col1, node.col2;
```
Can you help me in how can I create a Spanner Graph query which uses Approximate Nearest Neighbors vector index.

Thanks!