Cancel task functionality

[irgolic]

As runner_service is implemented with huggingface pipelines, this is dependent on huggingface/diffusers#374 being resolved.

That said, pipeline callbacks just got merged (huggingface/diffusers#521). It should be possible to stop the pipeline somehow from a callback.

Fulfiling this PR means to:

• add DEL /task/{task_id} endpoint
• on every step (in pipeline callback), poll to see if it's been cancelled
• if so, stop image generation, and send AbortedEvent(reason="cancelled"), unless generation had just finished, in which case send a FinishedEvent anyway