IAMR not supported

I can't connect to an IAMR redshift user with this lib

```sql
CREATE USER "IAMR:test_to_delete_redshift_ci" PASSWORD DISABLE;
```

but It work when I manually use boto3+psycopg2

```python

import os

import boto3
import psycopg2
import redshift_connector

os.environ["AWS_PROFILE"] = "redshift_test_role_2"

def with_lib():
conn = redshift_connector.connect(
iam=True,
cluster_identifier="YYYYYY",
database="AAAAA",
db_user="test_to_delete_redshift_ci",
password="",
region="eu-west-1",
)
cursor = conn.cursor()
cursor.execute("SELECT 10,20")
print(cursor.fetchall())

def manually():

RS_PORT = 5439
DATABASE = 'AAAAAA'
CLUSTER_ID = 'YYYYYYY'
RS_HOST = 'XXXXXXXXXXX'

client = boto3.client('redshift')

cluster_creds = client.get_cluster_credentials_with_iam(ClusterIdentifier=CLUSTER_ID)

print(cluster_creds)
conn = psycopg2.connect(
host=RS_HOST,
port=RS_PORT,
user=cluster_creds['DbUser'],
password=cluster_creds['DbPassword'],
database=DATABASE
)
cur = conn.cursor()
cur.execute("SELECT 10,20")

print(cur.fetchall())

if __name__ == '__main__':

with_lib()
manually()
```

with_lib fail with

```log
redshift_connector.error.ProgrammingError: {'S': 'FATAL', 'C': '42704', 'M': 'user "IAM:test_to_delete_redshift_ci" does not exist', 'F': '/opt/brazil-pkg-cache/packages/RedshiftPADB/RedshiftPADB-1.0.7910.0/AL2_x86_64/generic-flavor/src/src/pg/src/backend/utils/init/miscinit.c', 'L': '976', 'R': 'InitializeSessionUserId'}
```

Start by running with_lib() and manually() from the issue example and compare the generated user and connection behavior. Trace the connector's IAM authentication entry point, then verify that an IAMR user connects successfully and that SELECT 10,20 returns the expected rows.

由索引模型根据 Issue 内容生成。