WIP: add stability-ai image generation in workflow

[deependujha]

Description

fixes #693

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• New feature (non-breaking change which adds functionality)
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[deependujha]

I tried returning opencv image, numpy array and bytes. All of them give me error:

{
  "error": "Requested Workflow output serialization, but for output `stability_ai_image_generation` which evaluates into Python type: <class 'numpy.ndarray'> cannot successfully apply any of registered serializers.",
  "inner_error": "None"
}

I tried creating a WorkflowImageData with parentMetdata with random UUID. But, even this gives error:

{
  "error": "Requested Workflow output serialization, but for output `stability_ai_image_generation` which evaluates into Python type: <class 'inference.core.workflows.execution_engine.entities.base.WorkflowImageData'> cannot successfully apply any of registered serializers.",
  "inner_error": "None"
}