docs: add direct asyncpg IAM authentication tests

[jackwotherspoon]

Add direct TCP IAM authentication samples for asyncpg driver (SQLAlchemy + native asyncpg).

Should result in following sample in docs:

alloydb_sqlalchemy_asyncpg_connect_iam_authn_direct region tag:

import sqlalchemy
from sqlalchemy.ext.asyncio import create_async_engine
from sqlalchemy import event

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

# initialize Google Auth credentials
credentials, _ = 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 = create_async_engine(
    # Equivalent URL:
    # postgresql+asyncpg://<user>:empty@<host>:5432/<db_name>
    sqlalchemy.engine.url.URL.create(
        drivername="postgresql+asyncpg",
        username=user,  # IAM db user, e.g. service-account@project-id.iam
        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={"ssl": "require"},
)

# set 'do_connect' event listener to replace password with OAuth2 token
# must use engine.sync_engine as async events are not implemented
@event.listens_for(engine.sync_engine, "do_connect")
def auto_iam_authentication(dialect, conn_rec, cargs, cparams) -> None:
    cparams["password"] = get_authentication_token(credentials)

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

alloydb_native_asyncpg_connect_iam_authn_direct region tag:

import asyncpg

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

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

def get_authentication_token() -> 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

# ... inside async context
async with asyncpg.create_pool(
    user=user,  # IAM db user, e.g. service-account@project-id.iam
    password=get_authentication_token,  # set OAuth2 token as password
    host=ip_address,  # AlloyDB instance IP address
    port=5432,
    database=db,  # my-database
    ssl="require",
) as pool:
    # acquire connection from native asyncpg connection pool
    async with pool.acquire() as conn:
        time = await conn.fetchrow("SELECT NOW()")
        print("Current time is ", time[0])

Closes #120