This pull request introduces a new feature to support SQS visibility heartbeat in the machinery project. The main changes include:

• New Configuration Option: Added VisibilityHeartBeat to enable/disable the visibility heartbeat.
• Mock Implementation: Created a mock for sqsiface.SQSAPI using testify/mock.
• Unit Tests: Added tests to assert the number of calls to ChangeMessageVisibility.
• Code Refactoring: Refactored code to integrate the new feature.

Changes

• Added VisibilityHeartBeat configuration option.
• Implemented MockSQSAPI for testing.
• Updated unit tests.
• Refactored codebase.

Testing

• Verified ChangeMessageVisibility method calls.
• Ensured all existing tests pass.

Documentation

• Updated configuration documentation to include VisibilityHeartBeat.

This feature aims to improve message processing reliability by periodically extending the visibility timeout of in-flight messages, preventing other consumers from processing the same message simultaneously and avoiding duplicate consumption.