Implement a few command stubs

[AlekSi]

Readiness checklist

• I added/updated unit tests.
• I added/updated integration/compatibility tests.
• 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), Labels, Project and project's Sprint fields.
• I marked all done items in this checklist.

[codecov]

Codecov Report

Merging #2777 (e9785e1) into main (1d6997b) will decrease coverage by 36.97%.
The diff coverage is 1.35%.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2777       +/-   ##
===========================================
- Coverage   63.01%   26.05%   -36.97%     
===========================================
  Files         441      441               
  Lines       22689    22727       +38     
===========================================
- Hits        14297     5921     -8376     
- Misses       7461    16195     +8734     
+ Partials      931      611      -320     

Impacted Files	Coverage Δ
internal/handlers/pg/msg_explain.go	0.00% <0.00%> (-78.88%)	⬇️
internal/handlers/sqlite/cmd_query.go	0.00% <0.00%> (ø)
internal/handlers/sqlite/msg_createindexes.go	0.00% <0.00%> (ø)
internal/handlers/sqlite/msg_explain.go	0.00% <0.00%> (ø)
internal/handlers/sqlite/msg_hello.go	0.00% <ø> (ø)
internal/handlers/sqlite/msg_ismaster.go	0.00% <ø> (ø)
internal/util/testutil/path.go	69.84% <0.00%> (-23.81%)	⬇️
internal/util/testutil/db.go	87.30% <100.00%> (ø)

... and 142 files with indirect coverage changes

Flag	Coverage Δ
integration	4.94% <1.35%> (-51.54%)	⬇️
mongodb	4.94% <1.35%> (-0.01%)	⬇️
pg	?
unit	24.52% <1.35%> (-0.11%)	⬇️

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[w84thesun]

LGTM