Add MySQL backend registry

[adetunjii]

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 #3850 (751dddf) into main (cd22960) will decrease coverage by 0.81%.
The diff coverage is 44.91%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3850      +/-   ##
==========================================
- Coverage   76.20%   75.39%   -0.81%     
==========================================
  Files         317      323       +6     
  Lines       24027    24625     +598     
==========================================
+ Hits        18309    18567     +258     
- Misses       4655     4964     +309     
- Partials     1063     1094      +31     

Files	Coverage Δ
internal/backends/mysql/metadata/pool/uri.go	76.00% <100.00%> (ø)
internal/util/testutil/mysql.go	83.33% <100.00%> (+83.33%)	⬆️
internal/backends/mysql/metadata/metadata.go	0.00% <0.00%> (ø)
internal/backends/mysql/metadata/indexes.go	0.00% <0.00%> (ø)
internal/backends/mysql/metadata/pool/pool.go	57.29% <48.27%> (ø)
internal/backends/mysql/metadata/registry.go	47.66% <47.66%> (ø)

... and 14 files with indirect coverage changes

Flag	Coverage Δ
filter-true	72.28% <0.00%> (-0.22%)	⬇️
hana-1	?
integration	72.28% <0.00%> (-0.22%)	⬇️
mongodb-1	5.27% <0.00%> (ø)
postgresql-1	52.23% <0.00%> (-0.02%)	⬇️
postgresql-2	50.77% <0.00%> (+0.06%)	⬆️
postgresql-3	53.46% <0.00%> (ø)
sqlite-1	51.48% <0.00%> (-0.02%)	⬇️
sqlite-2	49.78% <0.00%> (ø)
sqlite-3	?
unit	30.89% <44.91%> (+0.43%)	⬆️

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

[AlekSi]

Test fails