[fix] Allow setting sourceType in config file

[alpreu]

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This change is a trivial rework / code cleanup without any test coverage.

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This change added tests and can be verified as follows:

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• Added integration tests for end-to-end deployment with large payloads (10MB)
• Extended integration test for recovery after broker failure

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• Dependencies (add or upgrade a dependency)
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• The schema
• The default values of configurations
• The threading model
• The binary protocol
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• The admin CLI options
• The metrics
• Anything that affects deployment

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