Support pointer in aggregate query

[dplewis]

Closes: #4489

Lints Postgres Adapter

const pipeline = [
   { group: { objectId: '$pointerField' } }
];

[codecov]

Codecov Report

Merging #4493 into master will decrease coverage by 0.02%.
The diff coverage is 92.85%.

@@            Coverage Diff             @@
##           master    #4493      +/-   ##
==========================================
- Coverage    92.8%   92.78%   -0.03%     
==========================================
  Files         118      118              
  Lines        8384     8394      +10     
==========================================
+ Hits         7781     7788       +7     
- Misses        603      606       +3

Impacted Files	Coverage Δ
src/Adapters/Storage/Mongo/MongoStorageAdapter.js	95.93% <100%> (+0.14%)	⬆️
...dapters/Storage/Postgres/PostgresStorageAdapter.js	97.01% <75%> (-0.1%)	⬇️
src/RestWrite.js	93.28% <0%> (-0.37%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update cb8f038...5c24811. Read the comment docs.