feat: make af-device name optional

[tlambert03]

closes #141 by allowing both autofocus device name and offset to be None.
If name is not provided, engines are allowed to simply "apply the offset" as best fits the current hardware (usually, there is only one way to do it at any given time anyway). If offset is None, then the presence of a Hardware Autofocus event simply means "turn on the autofocus without worrying about offset".

see related pymmcore-plus/pymmcore-plus#270

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (17e6533) 98.28% compared to head (da4c3a6) 98.28%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #145   +/-   ##
=======================================
  Coverage   98.28%   98.28%           
=======================================
  Files          14       14           
  Lines         877      877           
=======================================
  Hits          862      862           
  Misses         15       15           

Files	Coverage Δ
src/useq/_actions.py	100.00% <100.00%> (ø)
src/useq/_hardware_autofocus.py	100.00% <100.00%> (ø)

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[fdrgsp]

Love it!!!