Add a skip_missing kwarg to the as_ methods of Struct objects

[puddly]

This will be used to generate partial matches for device automation triggers in quirks, without having to define separate constants for "step" or "step_mode":

>>> from zigpy.zcl.clusters.general import LevelControl
>>> step = LevelControl.commands_by_name["step"].schema  # this should be improved
>>> step(step_mode=0).as_dict(skip_missing=True)
{'step_mode': <StepMode.Up: 0>}

[codecov-commenter]

Codecov Report

Merging #954 (89038e5) into dev (29f5268) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##              dev     #954   +/-   ##
=======================================
  Coverage   99.74%   99.74%           
=======================================
  Files          45       45           
  Lines        6640     6646    +6     
=======================================
+ Hits         6623     6629    +6     
  Misses         17       17           

Impacted Files	Coverage Δ
zigpy/types/struct.py	100.00% <100.00%> (ø)

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[coveralls]

Coverage increased (+0.0002%) to 99.737% when pulling 89038e5 on puddly:puddly/struct-dict-without-None into 29f5268 on zigpy:dev.