add `validate_always` flag to validate default values

[PrettyWood]

⚠️ WIP ⚠️

@samuelcolvin feedback welcome on this. IMO it will be useful in some usecases, in particular computed fields (I added a test to showcase this)

[codecov-commenter]

Codecov Report

Merging #230 (4d01dee) into main (6d431dc) will decrease coverage by 0.02%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #230      +/-   ##
==========================================
- Coverage   98.69%   98.66%   -0.03%     
==========================================
  Files          48       48              
  Lines        5042     5080      +38     
  Branches       35       35              
==========================================
+ Hits         4976     5012      +36     
- Misses         66       68       +2     

Impacted Files	Coverage Δ
src/errors/kinds.rs	99.15% <ø> (ø)
pydantic_core/_types.py	100.00% <100.00%> (ø)
src/validators/typed_dict.rs	98.96% <100.00%> (+0.06%)	:arrow_up:
src/input/datetime.rs	97.84% <0.00%> (-0.31%)	:arrow_down:
src/input/input_python.rs	98.40% <0.00%> (-0.27%)	:arrow_down:
src/lookup_key.rs	98.25% <0.00%> (-0.04%)	:arrow_down:
src/validators/function.rs	99.42% <0.00%> (-0.01%)	:arrow_down:
src/input/mod.rs	100.00% <0.00%> (ø)
... and 2 more

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[samuelcolvin]

replaced by #479