Implement authentication for PostgreSQL

[AlekSi]

Description

Closes #1694.

Readiness checklist

• I added some tests. More will be needed in Rework integration test setup and auth #1568.
• I made spot refactorings.
• I updated user documentation – in the next PR.
• I ran task all, and it passed.
• I added/updated comments for both exported and unexported top-level declarations (functions, types, etc).
• I checked comments rendering with task godocs.
• I ensured that the title is good enough for the changelog.
• (for maintainers only) I set Reviewers (@FerretDB/core), Assignee, Labels, Project and project's Sprint fields.
• I marked all done items in this checklist.

[vercel]

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Name	Status	Preview	Updated
ferret-db	✅ Ready (Inspect)	Visit Preview	Dec 30, 2022 at 1:25PM (UTC)

[codecov]

Codecov Report

Merging #1725 (34b1c27) into main (47e2246) will decrease coverage by 0.24%.
The diff coverage is 60.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1725      +/-   ##
==========================================
- Coverage   69.29%   69.05%   -0.25%     
==========================================
  Files         301      301              
  Lines       14233    14300      +67     
==========================================
+ Hits         9863     9875      +12     
- Misses       3435     3492      +57     
+ Partials      935      933       -2     

Impacted Files	Coverage Δ
internal/handlers/common/error.go	73.33% <ø> (ø)
internal/handlers/common/errorcode_string.go	80.00% <ø> (ø)
internal/handlers/pg/msg_explain.go	56.52% <ø> (-0.06%)	⬇️
internal/handlers/pg/msg_getparameter.go	80.45% <ø> (-2.02%)	⬇️
internal/handlers/pg/msg_saslstart.go	0.00% <0.00%> (-100.00%)	⬇️
internal/handlers/tigris/msg_delete.go	68.32% <0.00%> (-0.43%)	⬇️
internal/handlers/tigris/msg_explain.go	61.11% <ø> (-5.02%)	⬇️
internal/handlers/tigris/msg_insert.go	74.44% <0.00%> (-0.84%)	⬇️
internal/util/ctxutil/ctxutil.go	90.47% <ø> (ø)
internal/util/must/must.go	42.85% <ø> (ø)
... and 47 more

Flag	Coverage Δ
integration	64.44% <59.20%> (-0.60%)	⬇️
mongodb	5.97% <18.40%> (-8.54%)	⬇️
pg	53.03% <54.40%> (-1.01%)	⬇️
tigris	38.88% <40.80%> (-0.14%)	⬇️
unit	28.96% <13.48%> (-0.18%)	⬇️

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

[rumyantseva]

The code makes sense to me for now.
I tried things locally, seems to be working.

[b1ron]

All seems to be working. I tried to get mongo-test working with TLS but I kept receiving an invalid payload error. However, we probably won't ever need to run it with TLS.

[rumyantseva]

I didn't check super attentively, but the PR looks good to me overall, and things seem to be fixed.