Implement Query in new PostgreSQL backend

[chilagrow]

Description

Closes #3388.

Query tests for test-integration-postgresql fail now #3411 (comment), I think I need to implement insert.

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 #3411 (f194cb1) into main (d976d8c) will decrease coverage by 0.31%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main    #3411      +/-   ##
==========================================
- Coverage   74.62%   74.31%   -0.31%     
==========================================
  Files         414      415       +1     
  Lines       25766    25859      +93     
==========================================
- Hits        19227    19217      -10     
- Misses       5398     5497      +99     
- Partials     1141     1145       +4     

Files Changed	Coverage
internal/backends/postgresql/backend.go	0.00%
internal/backends/postgresql/collection.go	0.00%
internal/backends/postgresql/database.go	0.00%
internal/backends/postgresql/query_iterator.go	0.00%

Flag	Coverage Δ
filter-true	69.76% <0.00%> (-0.29%)	⬇️
hana-1	?
hana-2	?
hana-3	?
integration	69.76% <0.00%> (-0.29%)	⬇️
mongodb-1	4.67% <0.00%> (-0.02%)	⬇️
pg-1	42.99% <0.00%> (-0.16%)	⬇️
pg-2	42.72% <0.00%> (-0.16%)	⬇️
pg-3	43.17% <0.00%> (-0.26%)	⬇️
postgresql-1	∅ <ø> (∅)
postgresql-2	∅ <ø> (∅)
postgresql-3	∅ <ø> (∅)
sort-false	69.76% <0.00%> (-0.29%)	⬇️
sqlite-1	41.87% <0.00%> (-0.21%)	⬇️
sqlite-2	41.38% <0.00%> (-0.16%)	⬇️
sqlite-3	42.21% <0.00%> (-0.27%)	⬇️
unit	24.58% <0.00%> (-0.10%)	⬇️

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

[chilagrow]

Okay current implementation will not pass Query tests for test-integration-postgresql 😞

Integration test inserts documents using old pg backend, so documents are stored in the database but metadata in new postgresql is not updated and kept empty. When attempt to query, it returns empty document.
It's because insert in new postgresql backend does nothing.

I could:

1. implement insert in new backend and make integration test use new backend for insertion when use-new-pg flag is set.
2. fetch metadata from db every call in new backend
3. leave as it is for now, knowing tests fail (pushdown assertion also fails)

[AlekSi]

Let's do nothing for now

[rumyantseva]

LGTM!