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'}
```