Epic: Image generation

[abigail-hampson]

Plan ways to integrate image generation in to wagtail-ai, e.g.

• Automatically generating an image for an article by automatically parsing the title/content
• Generating alternative versions of uploaded images e.g. "generate a sketch version of this image"
• Generate a collage of this set of images

To consider:

• How we talk to image generation APIs. Currently not possible with the llm backend.
• Handling slow image generation processes
• Working with open source models

### Tasks
- [ ] https://github.com/wagtail/wagtail-ai/issues/34