[Vertex AI] Add ImageGenerationResponse for decoding PredictResponse

[andrewheard]

Added a type ImageGenerationResponse for decoding the PredictResponse from an image generation request.

Added ImagenImage and ImagenImageRepresentable, which are analogous to Part and PartsRepresentable for GenerativeModel. The types we vend in the public API will conform to ImagenImageRepresentable, allowing them to be used as input to other operations like image upscaling and image editing. Additionally, we can make system types like UIImage or CIImage conform to ImagenImageRepresentable, like PartsRepresentable, to allow these types as input in the future.

Note: Naming is all to be TBD.

#no-changelog

[google-oss-bot]

	1 Warning
⚠️	New public headers were added, did you remember to add them to the umbrella header?

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