Add subset functionality

[cthoyt]

BaseResolver.subresolver() allows for the resolver to be subsetted based on a list of keys. This might be useful for HPO scenarios.

[codecov-commenter]

Codecov Report

Merging #45 (896b42c) into main (bbb9cb3) will decrease coverage by 0.51%. The diff coverage is 83.33%.

@@            Coverage Diff             @@
##             main      #45      +/-   ##
==========================================
- Coverage   98.13%   97.63%   -0.51%     
==========================================
  Files          11       11              
  Lines         376      381       +5     
  Branches       62       63       +1     
==========================================
+ Hits          369      372       +3     
- Misses          3        5       +2     
  Partials        4        4              

Impacted Files	Coverage Δ
src/class_resolver/base.py	95.90% <81.81%> (-1.40%)	:arrow_down:
src/class_resolver/func.py	100.00% <100.00%> (ø)

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