[Vertex AI] Replace ImagenImage protocol with _ImagenImage struct

[andrewheard]

• Replaced the public ImagenImage protocol with a concrete _ImagenImage struct.
  • Unlike a protocol, a struct can be public while having internal properties.
    • This would still allow us to add an internal error property to accomplish the same thing as ErrorPart in GenerativeModel, allowing us to make UIImage, NSImage, etc., conform to ImagenImageRepresentable (a fail-able operation) in the future when we support image upscaling and/or editing that require images as input.
• Prefixed the imagenImage property in ImagenImageRepresentable with an underscore (i.e., _imagenImage).
  • This will discourage 3P use (developers only need to interact with the ImagenInlineDataImage and ImagenFileDataImage types) and hides the symbols from DocC generated documentation.
  • Apple uses this approach in the Swift Standard Library, e.g., https://github.com/swiftlang/swift/blob/0c860f8abaa76155358bf26779c9f1b93508438a/stdlib/public/core/UTF16.swift#L376
• Although ImagenImage contains no private information, it could result in confusion over whether it should be used. Together these changes should make it clear that its use is discouraged / unnecessary.

#14221
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