feat: DataSubclassMixin

[msto]

Per Slack discussion, it'd be helpful to have a generally reusable alternate constructor for subclassed dataclasses.

I chose to implement as a mixin. I think we probably want to always have this available, so I could see an argument for defining an alternate dataclass decorator that calls dataclasses.dataclass and injects this class method, to save a bit of verbosity, but this is a simple solution.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.39%. Comparing base (c13d3cc) to head (60a63a6).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #185      +/-   ##
==========================================
+ Coverage   89.35%   89.39%   +0.04%     
==========================================
  Files          18       18              
  Lines        2094     2103       +9     
  Branches      464      467       +3     
==========================================
+ Hits         1871     1880       +9     
  Misses        146      146              
  Partials       77       77              

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[clintval]

I would use this!!