Add MySQL backend

[adetunjii]

Description

Closes #3413

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 0% with 67 lines in your changes are missing coverage. Please review.

Project coverage is 73.98%. Comparing base (1615d26) to head (28c0e5d).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4137      +/-   ##
==========================================
- Coverage   74.71%   73.98%   -0.74%     
==========================================
  Files         353      353              
  Lines       22616    22675      +59     
==========================================
- Hits        16897    16775     -122     
- Misses       4431     4630     +199     
+ Partials     1288     1270      -18     

Files	Coverage Δ
internal/backends/mysql/insert.go	0.00% <0.00%> (ø)
internal/backends/mysql/collection.go	0.00% <0.00%> (ø)
internal/util/fsql/db.go	80.88% <0.00%> (-3.74%)	⬇️
internal/backends/mysql/backend.go	0.00% <0.00%> (ø)

... and 12 files with indirect coverage changes

Flag	Coverage Δ
filter-true	67.39% <0.00%> (-0.82%)	⬇️
hana-1	?
integration	67.39% <0.00%> (-0.82%)	⬇️
mongodb-1	4.75% <0.00%> (-0.02%)	⬇️
postgresql-1	48.22% <0.00%> (-0.23%)	⬇️
postgresql-2	48.72% <0.00%> (-0.14%)	⬇️
postgresql-3	49.22% <0.00%> (-0.14%)	⬇️
sqlite-1	47.53% <0.00%> (-0.13%)	⬇️
sqlite-2	47.91% <0.00%> (-0.14%)	⬇️
sqlite-3	48.31% <0.00%> (-0.09%)	⬇️
unit	32.66% <0.00%> (-0.09%)	⬇️

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

[chilagrow]

Thanks for your great work 🤗