docs: add direct psycopg2 IAM authentication test

[jackwotherspoon]

Adding sample for direct TCP connection for psycopg2 with automatic IAM database authentication through the use of SQLAlchemy's do_connect event listener.

Region tags should result in the following code sample:

import sqlalchemy
from sqlalchemy import event

import google.auth
from google.auth.credentials import Credentials
from google.auth.transport.requests import Request

# initialize Google Auth creds
creds, _ = google.auth.default(
    scopes=["https://www.googleapis.com/auth/cloud-platform"]
)

def get_authentication_token(credentials: Credentials) -> str:
    """Get OAuth2 access token to be used for IAM database authentication"""
    # refresh credentials if expired
    if not credentials.valid:
        request = Request()
        credentials.refresh(request)
    return credentials.token

engine = sqlalchemy.create_engine(
    # Equivalent URL:
    # postgresql+psycopg2://<user>:empty@<host>:<port>/<db_name>
    sqlalchemy.engine.url.URL.create(
        drivername="postgresql+psycopg2",
        username=user,  # IAM db user, [email protected]
        password="",  # placeholder to be replaced with OAuth2 token
        host=ip_address,  # AlloyDB instance IP address
        port=5432,
        database=db_name,  # "my-database-name"
    ),
    connect_args={"sslmode": "require"},
)

# set 'do_connect' event listener to replace password with OAuth2 token
@event.listens_for(engine, "do_connect")
def auto_iam_authentication(dialect, conn_rec, cargs, cparams) -> None:
    cparams["password"] = get_authentication_token(creds)

# use connection from connection pool to query AlloyDB database
with engine.connect() as conn:
    time = conn.execute(sqlalchemy.text("SELECT NOW()")).fetchone()
    conn.commit()
    print("Current time is ", time[0])