Implement $first accumulator

[adetunjii]

Closes #2128.

Readiness checklist

• I added/updated unit tests (and they pass).
• I added/updated integration/compatibility tests (and they pass).
• I added/updated comments and checked rendering.
• I made spot refactorings.
• I updated user documentation.
• I ran task all, and it passed.
• I ensured that PR title is good enough for the changelog.
• (for maintainers only) I set Reviewers (@FerretDB/core), Milestone (Next), Labels, Project and project's Sprint fields.
• I marked all done items in this checklist.

[codecov]

Codecov Report

Merging #3792 (5ab8dee) into main (90ca808) will decrease coverage by 0.27%.
The diff coverage is 0.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3792      +/-   ##
==========================================
- Coverage   76.39%   76.13%   -0.27%     
==========================================
  Files         317      319       +2     
  Lines       23121    23190      +69     
==========================================
- Hits        17663    17655       -8     
- Misses       4422     4501      +79     
+ Partials     1036     1034       -2     

Files	Coverage Δ
...ggregations/operators/accumulators/accumulators.go	84.78% <ø> (ø)
...handler/common/aggregations/operators/operators.go	81.25% <ø> (ø)
...nal/handler/common/aggregations/operators/first.go	0.00% <0.00%> (ø)
...ommon/aggregations/operators/accumulators/first.go	0.00% <0.00%> (ø)

... and 7 files with indirect coverage changes

Flag	Coverage Δ
filter-true	72.20% <0.00%> (-0.25%)	⬇️
hana-1	?
integration	72.20% <0.00%> (-0.25%)	⬇️
mongodb-1	5.46% <0.00%> (-0.02%)	⬇️
postgresql-1	50.88% <0.00%> (-0.15%)	⬇️
postgresql-2	52.36% <0.00%> (-0.04%)	⬇️
postgresql-3	51.78% <0.00%> (-0.09%)	⬇️
sqlite-1	50.15% <0.00%> (-0.15%)	⬇️
sqlite-2	51.39% <0.00%> (-0.27%)	⬇️
sqlite-3	50.87% <0.00%> (-0.10%)	⬇️
unit	30.14% <0.00%> (-0.10%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

[panga]

Any updates on this fix?

[adetunjii]

Any updates on this fix?

No, none at the moment. I had to stop working on this in the meantime. This is majorly due to internal architecture considerations.

[Evengard]

Man, this is a stopper for me for now... That is sad. My app almost ran correctly xD