Implement Collection.Compact for PostgreSQL

[rumyantseva]

Description

Closes #3484.

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 #3603 (0bf22e5) into main (0d9e48c) will decrease coverage by 0.66%.
The diff coverage is 46.66%.

@@            Coverage Diff             @@
##             main    #3603      +/-   ##
==========================================
- Coverage   61.44%   60.78%   -0.66%     
==========================================
  Files         426      426              
  Lines       27473    27502      +29     
==========================================
- Hits        16880    16717     -163     
- Misses       9583     9759     +176     
- Partials     1010     1026      +16     

Files	Coverage Δ
internal/backends/postgresql/collection.go	63.68% <46.66%> (-0.93%)	⬇️

... and 22 files with indirect coverage changes

Flag	Coverage Δ
filter-true	57.10% <0.00%> (-0.87%)	⬇️
hana-1	?
integration	57.10% <0.00%> (-0.87%)	⬇️
mongodb-1	4.49% <0.00%> (?)
postgresql-1	41.07% <0.00%> (-0.06%)	⬇️
postgresql-2	41.65% <0.00%> (-0.05%)	⬇️
postgresql-3	39.91% <0.00%> (-0.09%)	⬇️
sort-false	57.10% <0.00%> (-0.87%)	⬇️
sqlite-1	?
sqlite-2	41.82% <0.00%> (-0.05%)	⬇️
sqlite-3	?
unit	22.49% <46.66%> (+0.11%)	⬆️

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

[chilagrow]

LGTM

[noisersup]

LGTM