Add FERRETDB_USERNAME stub

[noisersup]

Description

Closes #4245.

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

All modified and coverable lines are covered by tests ✅

Project coverage is 74.06%. Comparing base (6e3f7b2) to head (fb05ef2).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4374      +/-   ##
==========================================
- Coverage   74.80%   74.06%   -0.74%     
==========================================
  Files         326      326              
  Lines       22471    22471              
==========================================
- Hits        16809    16643     -166     
- Misses       4429     4617     +188     
+ Partials     1233     1211      -22     

see 14 files with indirect coverage changes

Flag	Coverage Δ
filter-true	67.42% <ø> (-0.83%)	⬇️
hana-1	?
integration	67.42% <ø> (-0.83%)	⬇️
mongodb-1	5.09% <ø> (-0.01%)	⬇️
postgresql-1	44.71% <ø> (+0.09%)	⬆️
postgresql-2	44.79% <ø> (+0.01%)	⬆️
postgresql-3	41.44% <ø> (ø)
postgresql-4	41.07% <ø> (+<0.01%)	⬆️
postgresql-5	43.25% <ø> (ø)
sqlite-1	43.72% <ø> (ø)
sqlite-2	44.12% <ø> (-0.03%)	⬇️
sqlite-3	40.69% <ø> (ø)
sqlite-4	40.42% <ø> (-0.03%)	⬇️
sqlite-5	42.38% <ø> (ø)
unit	33.11% <ø> (ø)

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

[AlekSi]

I think FERRETDB_SETUP_USERNAME is already handled by FerretDB via the usual Kong CLI

[noisersup]

I think FERRETDB_SETUP_USERNAME is already handled by FerretDB via the usual Kong CLI

@AlekSi What's the goal of this issue then? It's possible to set setup username in docker already.

[AlekSi]

We should check that it works

[noisersup]

It works, I've already checked that while working on #4246. Should I write unit test for that?

[AlekSi]

No, I don't think there is a need for it