Extract user creation and move to backends package

[b1ron]

Description

Closes #4243.

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

Attention: Patch coverage is 88.00000% with 15 lines in your changes are missing coverage. Please review.

Project coverage is 74.19%. Comparing base (23d1e94) to head (5e31a54).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4248      +/-   ##
==========================================
- Coverage   74.90%   74.19%   -0.72%     
==========================================
  Files         323      325       +2     
  Lines       22365    22414      +49     
==========================================
- Hits        16752    16629     -123     
- Misses       4385     4575     +190     
+ Partials     1228     1210      -18     

Files	Coverage Δ
internal/handler/authenticate.go	76.47% <100.00%> (ø)
internal/util/password/password.go	100.00% <100.00%> (ø)
internal/handler/msg_createuser.go	75.22% <97.56%> (-0.31%)	⬇️
internal/handler/msg_updateuser.go	64.00% <92.00%> (+3.42%)	⬆️
internal/backends/user.go	77.77% <77.77%> (ø)

... and 15 files with indirect coverage changes

Flag	Coverage Δ
filter-true	67.48% <88.00%> (-0.79%)	⬇️
hana-1	?
integration	67.48% <88.00%> (-0.79%)	⬇️
mongodb-1	5.13% <0.00%> (-0.03%)	⬇️
postgresql-1	44.79% <66.40%> (+0.02%)	⬆️
postgresql-2	44.88% <48.00%> (-0.04%)	⬇️
postgresql-3	41.55% <46.40%> (-0.02%)	⬇️
postgresql-4	41.13% <46.40%> (-0.03%)	⬇️
postgresql-5	43.35% <69.60%> (-0.01%)	⬇️
sqlite-1	43.76% <66.40%> (-0.02%)	⬇️
sqlite-2	44.20% <48.00%> (+0.01%)	⬆️
sqlite-3	40.78% <46.40%> (-0.03%)	⬇️
sqlite-4	40.51% <46.40%> (-0.02%)	⬇️
sqlite-5	42.48% <69.60%> (+<0.01%)	⬆️
unit	33.22% <0.00%> (-0.08%)	⬇️

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

[b1ron]

Tentatively sending for review for feedback on current implementation.

Still needs linter fixes and some manual testing.

[chilagrow]

Tiny comment 🤗

[chilagrow]

Great!

[AlekSi]

Another reason is to try to hide values of that type from logs in the future.

That's experimental. I will either update more code to use that type or remove it in future PRs.

[mergify]

⚠️ The sha of the head commit of this PR conflicts with #4311. Mergify cannot evaluate rules on this PR. ⚠️