Consider making autofocus_device_name optional

[tlambert03]

I'm wondering whether the autofocus plan has leaked a bit too much implementation details into the model.

I think in the vast majority of cases:

• there will only be one (or no) autofocus device.
• in most cases, I'm guessing that that device will generally be "auto-detectable" by the engine consuming the event (for example, i suspect we could auto-determine what the autofocus device was for micro-manager in the engine for most if not all supported device adapters)

So, that leads me to wonder whether we should update the autofocus plan to be, at minimimum, nothing more than a boolean that says "do autofocus for these axes", allowing autofocus_device_name: str | None. If None, it would imply "do your best to find it".

This thought comes from looking at pymmcore-plus/pymmcore-widgets#201 and wishing we didn't need the user to enter/select a string device name