Add stubs for Collection.Compact

[AlekSi]

Description

Refs #75.

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 #3485 (2cd38b3) into main (dd6e9dd) will decrease coverage by 0.59%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main    #3485      +/-   ##
==========================================
- Coverage   74.68%   74.10%   -0.59%     
==========================================
  Files         422      422              
  Lines       26374    26393      +19     
==========================================
- Hits        19697    19558     -139     
- Misses       5544     5680     +136     
- Partials     1133     1155      +22     

Files	Coverage Δ
internal/backends/decorators/dummy/collection.go	0.00% <0.00%> (ø)
internal/backends/decorators/oplog/collection.go	0.00% <0.00%> (ø)
internal/backends/hana/collection.go	0.00% <0.00%> (ø)
internal/backends/postgresql/collection.go	18.02% <0.00%> (-0.32%)	⬇️
internal/backends/sqlite/collection.go	78.70% <0.00%> (-0.87%)	⬇️
internal/backends/collection.go	91.25% <0.00%> (-8.75%)	⬇️

... and 16 files with indirect coverage changes

Flag	Coverage Δ
filter-false	?
filter-true	69.10% <0.00%> (-0.65%)	⬇️
hana-1	?
integration	69.10% <0.00%> (-0.70%)	⬇️
mongodb-1	4.63% <0.00%> (-0.01%)	⬇️
pg-1	40.55% <0.00%> (-0.58%)	⬇️
pg-2	43.22% <0.00%> (-0.49%)	⬇️
pg-3	42.87% <0.00%> (-0.66%)	⬇️
postgresql-1	∅ <ø> (∅)
postgresql-2	∅ <ø> (∅)
postgresql-3	∅ <ø> (∅)
sort-false	69.10% <0.00%> (-0.28%)	⬇️
sort-true	?
sqlite-1	39.72% <0.00%> (-0.06%)	⬇️
sqlite-2	42.34% <0.00%> (-0.14%)	⬇️
sqlite-3	42.37% <0.00%> (-0.07%)	⬇️
unit	25.03% <0.00%> (-0.07%)	⬇️

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

[chilagrow]

Thank you, just one comment

[chilagrow]

Looks good to me 👍

[chilagrow]

LGTM 🚀