[Testing] Implement real backend mode

[dhegberg]

Migration from aws/aws-durable-execution-sdk-python-testing#29

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Context

Add configuration option for real backend vs in-memory mode

Technical Requirements

• Add configuration option for real backend vs in-memory mode
• Implement AWS service client integration for real backend mode
• Add credential handling and error management for AWS calls
• Ensure API consistency between modes

Acceptance Criteria

• Configuration option added for real backend vs in-memory mode
• AWS service client integration implemented for real backend mode
• Credential handling and error management added for AWS calls
• API consistency ensured between modes