Support 'external_id' for temporary credentials

## Driver version

v2.0.907 [Latest](https://github.com/aws/amazon-redshift-python-driver/releases/latest)

## Redshift version

N/A

## Client Operating System

N/A

## Python version

N/A

## Table schema

N/A

## Problem description

1. Expected behaviour: Library is able to use EKS IRSA / AWS SSO to discovery credential, then use `AssumeRole` with `role_arn` and `external_id` to receive current one.
2. Actual behaviour: No support for `external_id`.
4. Error message/stack trace: N/A
5. Any other details that can be helpful:

Partner Hosted Foundational Technical Review requires "CAA-002 - Use external ID with cross-account roles to access customer accounts."

Credential management is already supported. In my opinion, AWS SDK (in this scenario `boto3`) defines the credentials standard for IAM that is adopted for language in AWS. In my opinion, this library should only receive instance `boto3.Session` to build proper IAM credentials on its own. It allows use `aws-assume-role-lib` ( https://github.com/benkehoe/aws-assume-role-lib ) to automatically refresh credentials when reconnection happens (The fact that it is an external library is a separate problem in boto3 because [JavaScript](https://docs.aws.amazon.com/AWSJavaScriptSDK/latest/AWS/TemporaryCredentials.html) / Java ( https://docs.aws.amazon.com/AWSJavaSDK/latest/javadoc/com/amazonaws/auth/STSAssumeRoleSessionCredentialsProvider.html ) has the appropriate built-in credential provider.). Then a large amount of code - repeated in relation to boto3 - regarding the use of credential management will become redundant.

## Python Driver trace logs

## Reproduction code

```python
import redshift_connector

# Connects to Redshift cluster using AWS credentials
conn = redshift_connector.connect(
host='examplecluster.abc123xyz789.us-west-1.redshift.amazonaws.com',
database='dev',
user='awsuser',
role_arn='aws:...',
external_id='...'
)
```

CC: @podpio

Start at the redshift_connector.connect example and trace how role_arn credentials are obtained. Review the requested boto3.Session and AssumeRole flow, including external_id and credential refresh on reconnection. Done means the connector accepts external_id for temporary credentials and the documented reproduction flow works.

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