Update PostgresStore to support a connection pool

[tyler-ball]

I'm using Postgres for persistence in my project and I'm successfully using a connection pool like this:

pool = ConnectionPool(
    # Example configuration
    conninfo=settings.postgres.conn_string,
    max_size=20,
    kwargs={
        "autocommit": True,
        "prepare_threshold": 0,
        "row_factory": dict_row,
    },
)
self.checkpointer = PostgresSaver(conn=pool)
self.checkpointer.setup()

But it looks like the PostgresStore does support a connection pool, only a connection. I like using the pool over the .from_conn_string because then I don't have to wrap all my FastAPI function implementations - I just let the PostgresSaver make connections whenever it needs them.

I was going to look into updating the PostgresStore to support connection pool, copying the pattern from PostgresSaver, and your contributing policy said I should open a discussion first, so here I am! Thanks

[tyler-ball]

Pinging @bracesproul @eyurtsev since I saw yall making recent commits and the contributing doc said to tag a maintainer.

I started an implementation of this at #2475