Allow IAM Session Tokens for S3 cache client credentials

What does this MR do?

When using access keys for authentication, we currently provide the Minio S3 Client Credentials a hardcoded empty string for Session Token.

This MR extends the configuration by offering an additional input for specifying an AWS IAM Session Token if needed.

Why was this MR needed?

• Specifying only Access Key ID and Secret Key is fine when using long-lived IAM credentials because only the Access Key ID and Secret Key are issued.
• When operating in an environment that uses temporary IAM credentials (usually issued through STS IAM Role Assumptions), a Session Token also needs to be provided.

What's the best way to test this MR?

1. Issue temporary credentials for a role that has S3 Bucket permissions
   • aws sts assume-role --role-arn arn:aws:iam::123456789123:role/myAwesomeRole --role-session-name test --duration-seconds 900
2. Configure the runner using the printed credentials (including the session token)
   • Test a single build using the run-single command

Before this change (only access key id and secret key supplied)

After this change (being able to specify a session token too)

What are the relevant issue numbers?

• None, because it is a pretty small fix. I can create one if it's really necessary.