Add support for Django 5.1 PostgreSQL connection pools to env.db_url

[federicobond]

From the Django 5.1 release notes:

Django 5.1 also introduces connection pool support for PostgreSQL. As the time to establish a new connection can be relatively long, keeping connections open can reduce latency.

To use a connection pool with psycopg, you can set the "pool" option inside OPTIONS to be a dict to be passed to ConnectionPool, or to True to use the ConnectionPool defaults:

DATABASES = {
    "default": {
        "ENGINE": "django.db.backends.postgresql",
        # ...
        "OPTIONS": {
            "pool": {
                "min_size": 2,
                "max_size": 4,
                "timeout": 10,
            }
        },
    },
}

https://docs.djangoproject.com/en/5.1/releases/5.1/#postgresql-connection-pools

The format does not lend itself well to be passed as query params in the DATABASE_URL.

Perhaps they can be passed in a new extra_options argument?

env.db_url(extra_options={
  "pool": {"min_size": 2, "max_size": 4, "timeout": 10}
})