Add TLS support to proxy mode

[AlekSi]

Description

It allows us to see communication between TLS- and authentication-enabled client and server.

Tested manually with task run-proxy-secured and

docker compose exec mongodb mongosh --verbose --shell \
'mongodb://username:password@host.docker.internal:27018/?tls=true&tlsCertificateKeyFile=/etc/certs/client.pem&tlsCaFile=/etc/certs/rootCA-cert.pem'

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 #3783 (eaaceea) into main (90ca808) will decrease coverage by 0.12%.
The diff coverage is 40.84%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3783      +/-   ##
==========================================
- Coverage   76.39%   76.27%   -0.12%     
==========================================
  Files         317      318       +1     
  Lines       23121    23153      +32     
==========================================
- Hits        17663    17661       -2     
- Misses       4422     4456      +34     
  Partials     1036     1036              

Files	Coverage Δ
ferretdb/ferretdb.go	83.33% <100.00%> (+0.16%)	⬆️
internal/clientconn/listener.go	80.62% <100.00%> (+5.27%)	⬆️
internal/clientconn/conn.go	55.49% <0.00%> (ø)
internal/util/tlsutil/tlsutil.go	41.93% <41.93%> (ø)
internal/handler/proxy/proxy.go	0.00% <0.00%> (ø)

... and 8 files with indirect coverage changes

Flag	Coverage Δ
filter-true	72.33% <36.61%> (-0.12%)	⬇️
hana-1	?
integration	72.33% <36.61%> (-0.12%)	⬇️
mongodb-1	5.46% <0.00%> (-0.01%)	⬇️
postgresql-1	50.99% <36.61%> (-0.04%)	⬇️
postgresql-2	52.48% <36.61%> (+0.07%)	⬆️
postgresql-3	51.84% <36.61%> (-0.03%)	⬇️
sqlite-1	50.26% <36.61%> (-0.04%)	⬇️
sqlite-2	51.67% <36.61%> (+0.01%)	⬆️
sqlite-3	50.93% <36.61%> (-0.03%)	⬇️
unit	30.21% <25.35%> (-0.03%)	⬇️

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