Add transactions to fsql

[AlekSi]

Description

They will be covered by integration tests in the next PR.

Closes #2750.

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 #3278 (9ffa80a) into main (2e847f0) will decrease coverage by 0.31%.
The diff coverage is 1.83%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3278      +/-   ##
==========================================
- Coverage   75.24%   74.93%   -0.31%     
==========================================
  Files         399      400       +1     
  Lines       22681    22788     +107     
==========================================
+ Hits        17066    17076      +10     
- Misses       4625     4725     +100     
+ Partials      990      987       -3     

Files Changed	Coverage Δ
internal/util/fsql/tx.go	0.00% <0.00%> (ø)
internal/util/fsql/db.go	60.00% <2.50%> (-35.46%)	⬇️
internal/util/fsql/rows.go	100.00% <100.00%> (+14.28%)	⬆️

... and 2 files with indirect coverage changes

Flag	Coverage Δ
hana	?
integration	71.41% <1.83%> (-0.30%)	⬇️
mongodb	5.16% <0.00%> (-0.03%)	⬇️
pg	63.48% <0.00%> (-0.31%)	⬇️
shard-1	55.70% <1.83%> (-0.33%)	⬇️
shard-2	54.39% <1.83%> (-0.26%)	⬇️
shard-3	55.57% <1.83%> (-0.27%)	⬇️
sqlite	54.06% <1.83%> (-0.22%)	⬇️
unit	24.11% <1.83%> (-0.12%)	⬇️

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[chilagrow]

Looks great!