INTPYTHON-637 benchmarking test against in-memory and postgres

[caseyclements]

This PR adds a new test that runs a graph taken from /langgraph/bench/fanout_to_subgraph.py in the main langgraph repo.

It is in DRAFT status as it introduces psycopg as a dependency which we have to discuss.

The results for sync and async versions are shown below. (The actual numbers don't matter.) The mongodb numbers are equivalent running on either MongoDB or Atlas CLI locally.

tests/integration_tests/test_fanout_to_subgraph.py::test_sync

Begin test_sync
mongodb: 8.9449 seconds
postgres: 10.8356 seconds
in_memory: 3.4794 seconds
PASSED
tests/integration_tests/test_fanout_to_subgraph.py::test_async

Begin test_async
mongodb_async: 9.7151 seconds
postgres_async: 16.1918 seconds
in_memory_async: 2.6644 seconds
PASSED