feat: add entity write validation

[wschurman]

Why

#25

This adds the ability for custom field validators to be specified for fields, which will allow moving some field-specific logic to the entity layer in application in a structured way rather than needing the validation to be at the edge/API layer.

Some notes:

• Validating runtime types of field values is another logical extension here. This is straightforward for all fields except enums since enum membership validation isn't possible in typescript due to the way it's compiled. Object.values(enumType).includes(value) doesn't work due to reverse mapping generated https://www.typescriptlang.org/docs/handbook/enums.html#reverse-mappings, which would make keys valid values.
• Validating whole entities also seems like a good feature. https://guides.rubyonrails.org/active_record_validations.html may be a good superset of validations that would be worth investigating.

How

Add write validator and validate during mutation.

Test Plan

Run new tests.

[codecov]

Codecov Report

Merging #50 into master will increase coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #50      +/-   ##
==========================================
+ Coverage   93.91%   94.00%   +0.09%     
==========================================
  Files          57       57              
  Lines        1347     1368      +21     
  Branches      145      151       +6     
==========================================
+ Hits         1265     1286      +21     
  Misses         81       81              
  Partials        1        1              

Flag	Coverage Δ
#integration	94.00% <100.00%> (+0.09%)	⬆️
#unittest	94.00% <100.00%> (+0.09%)	⬆️

Impacted Files	Coverage Δ
packages/entity/src/EntityConfiguration.ts	100.00% <100.00%> (ø)
packages/entity/src/EntityFields.ts	100.00% <100.00%> (ø)
packages/entity/src/EntityMutator.ts	100.00% <100.00%> (ø)

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