feat: entity mutation validators

[quinlanj]

Why

Implements validation for entity mutations. Validators are specified by the user, and run before every create and update mutation. Validators are run before any triggers.

#25

Tests

• new unit tests pass (verifying validator call counts on different mutation operations)
• new integration tests pass (verifying validators work as expected inside a txn)

[codecov]

Codecov Report

Merging #67 into master will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #67      +/-   ##
==========================================
+ Coverage   94.55%   94.57%   +0.02%     
==========================================
  Files          59       59              
  Lines        1488     1494       +6     
  Branches      165      166       +1     
==========================================
+ Hits         1407     1413       +6     
  Misses         79       79              
  Partials        2        2              

Flag	Coverage Δ
#integration	94.57% <100.00%> (+0.02%)	⬆️
#unittest	94.57% <100.00%> (+0.02%)	⬆️

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Impacted Files	Coverage Δ
packages/entity/src/EntityCompanion.ts	100.00% <ø> (ø)
packages/entity/src/EntityCompanionProvider.ts	100.00% <100.00%> (ø)
packages/entity/src/EntityMutator.ts	98.02% <100.00%> (+0.03%)	⬆️
packages/entity/src/EntityMutatorFactory.ts	100.00% <100.00%> (ø)

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[wschurman]

We'll probably only want to run these on create and update, so that's what makes me think that there should probably just be one set of validators (see inline comment).

[wschurman]

LGTM. Just add some test (as mentioned in summary) and convert the PR to non-draft and I'll review more thoroughly.

[wschurman]

Before landing, update the summary (remove TODOs, detail behavior slightly more). Other than that, looks great. I'll probably rename EntityMutationTrigger to something else in a follow-up PR.