Added Support for Explicitly Assuming AWS IAM Roles

[brnquester]

Overview

This PR introduces a new feature that enables the application to explicitly assume an IAM role when interacting with AWS services.

This functionality supports two scenarios:

1. Default Credentials: If no key and secret are provided, the application will assume the specified role using default credentials, such as an IAM role attached to an EC2 instance.
2. Explicit Credentials: If key and secret are provided, the application will use the specified IAM user's credentials to assume the given role.

Key Changes:

1. Added support for the assume_role option in the configuration.
2. Implemented logic to handle role assumption using either default credentials or explicitly provided IAM user credentials.
3. Updated documentation to reflect the new functionality.