Implement distinct command

[rumyantseva]

Description

Closes #945.

Readiness checklist

• I added unit tests for new functionality or bug fixes.
• I added integration tests for new functionality or bug fixes.
• I added compatibility tests for new functionality or bug fixes.
• I made spot refactorings.
• I updated user documentation.
• I ran task all, and it passed.
• I added/updated comments for both exported and unexported top-level declarations (functions, types, etc).
• I checked comments rendering with task godocs.
• I ensured that the title is good enough for the changelog.
• (for maintainers only) I set Reviewers (@FerretDB/core), Assignee, Labels, Project and project's Sprint fields.
• I marked all done items in this checklist.

[vercel]

The latest updates on your projects. Learn more about Vercel for Git ↗︎

Name	Status	Preview	Updated
ferret-db	✅ Ready (Inspect)	Visit Preview	Jan 5, 2023 at 8:21AM (UTC)

[codecov]

Codecov Report

Merging #1739 (e1f0fb9) into main (654f728) will decrease coverage by 0.14%.
The diff coverage is 61.36%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1739      +/-   ##
==========================================
- Coverage   69.18%   69.03%   -0.15%     
==========================================
  Files         302      307       +5     
  Lines       14409    14585     +176     
==========================================
+ Hits         9969    10069     +100     
- Misses       3505     3552      +47     
- Partials      935      964      +29     

Impacted Files	Coverage Δ
internal/handlers/common/error.go	73.33% <ø> (ø)
internal/handlers/common/errorcode_string.go	80.00% <ø> (ø)
internal/handlers/common/msg_listcommands.go	100.00% <ø> (ø)
internal/handlers/common/sort_documents.go	91.56% <ø> (ø)
internal/handlers/dummy/msg_distinct.go	0.00% <0.00%> (ø)
internal/handlers/common/distinct.go	54.05% <54.05%> (ø)
internal/handlers/tigris/msg_distinct.go	65.71% <65.71%> (ø)
internal/handlers/pg/msg_distinct.go	65.90% <65.90%> (ø)
internal/handlers/common/sort_array.go	76.19% <76.19%> (ø)
build/version/version.go	63.01% <0.00%> (-4.11%)	⬇️
... and 6 more

Flag	Coverage Δ
integration	63.94% <61.36%> (-0.16%)	⬇️
mongodb	5.86% <0.00%> (-0.08%)	⬇️
pg	52.58% <48.29%> (-0.18%)	⬇️
tigris	40.80% <42.04%> (+0.29%)	⬆️
unit	29.08% <0.00%> (-0.25%)	⬇️

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

[chilagrow]

Looks good! Just one additional edge case test suggestion 🤗

[chilagrow]

Looks great

[noisersup]

I like the implementation, though I have one question related to it + a couple of more general questions. If we don't want to change it, I'm ready to approve it!