Progress update events

[irgolic]

Using diffusers pipeline callbacks, at every timestep (unless this drastically slows down computation), a progress update event should be emitted. As per a request parameter, a GeneratedImage should be included with the event every x timesteps, to show intermediate generation progress.

Fulfilling this PR means to:

• Add a ProgressEvent(percentage: float, image: Optional[GeneratedImage]) to EventUnion
• Add a progress_update_every_n_steps: int = 5 (or better worder) attribute to Param
• Implement a pipeline callback in runner_service that emits ProgressEvent every timestep, attaching an image every progress_update_every_n_steps

Something similar should probably be done for model download progress too...

[MicahZoltu]

When this is implemented, it would be good to do a bit of testing to figure out if extracting an intermediate image from the GPU slows down generation at all, or if its impact is only CPU/network that is wasted on doing so. Once the answer is known, these details can be included in any associated documentation to make it clear what the tradeoffs are.