feat: support for cancelling generations

[wbruna]

Adds an sd_cancel_generation function that can be called asynchronously to interrupt the current generation.

The log handling is still a bit rough on the edges, but I wanted to gather more feedback before polishing it. I've included a flag to allow finer control of what to cancel: everything, or keep and decode already-generated latents but cancel the current and next generations. Would an extra "finish the already started latent but cancel the batch" mode be useful? Or should I simplify it instead, keeping just the cancel-everything mode?

The function should be safe to be called from the progress or preview callbacks, a separate thread, or a signal handler. I've included a Unix signal handler on main.cpp just to be able to test it: the first Ctrl+C cancels the batch and the current gen, but still finishes the already generated latents, while a second Ctrl+C cancels everything (although it won't interrupt it in the middle of a generation step anymore).

fixes #1036