feat: add loadManyByIDsNullableAsync loader method

[wschurman]

Why

Found a use case for this method in our use of the entity library. Easy enough to implement.

How

Add method, add tests.

Test Plan

Run tests.

[codecov]

Codecov Report

Merging #214 (3e874f5) into main (d2a4405) will decrease coverage by 0.09%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##             main     #214      +/-   ##
==========================================
- Coverage   95.55%   95.46%   -0.09%     
==========================================
  Files          79       79              
  Lines        2068     2075       +7     
  Branches      279      258      -21     
==========================================
+ Hits         1976     1981       +5     
- Misses         91       93       +2     
  Partials        1        1              

Flag	Coverage Δ
integration	95.46% <71.42%> (-0.09%)	⬇️
unittest	95.46% <71.42%> (-0.09%)	⬇️

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Files	Coverage Δ
packages/entity/src/EnforcingEntityLoader.ts	100.00% <100.00%> (ø)
packages/entity/src/EntityLoader.ts	89.09% <50.00%> (-1.48%)	⬇️