feat: allow fields to be frozen and add strict + context support to validate_assignment

[PrettyWood]

fix #219

[codecov]

Codecov Report

Merging #221 (ede8506) into main (fdf42b8) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #221   +/-   ##
=======================================
  Coverage   98.66%   98.67%           
=======================================
  Files          48       48           
  Lines        5031     5043   +12     
  Branches       35       35           
=======================================
+ Hits         4964     4976   +12     
  Misses         67       67           

Impacted Files	Coverage Δ
src/errors/kinds.rs	99.15% <ø> (ø)
pydantic_core/_types.py	100.00% <100.00%> (ø)
src/validators/mod.rs	98.33% <100.00%> (+0.04%)	⬆️
src/validators/typed_dict.rs	98.89% <100.00%> (+<0.01%)	⬆️

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[samuelcolvin]

thanks so much.