tjson: Implement datetime

[noisersup]

Description

This PR closes #905.

Readiness checklist

• I added tests for new functionality or bugfixes.
• I ran task all, and it passed.
• I added/updated comments for both exported and unexported top-level declarations (functions, types, etc).
• I checked comments rendering with task godocs.
• (for maintainers only) I set Reviewers (@FerretDB/core), Assignee, Labels, Project and project's Sprint fields.
• I marked all done items in this checklist.

[codecov]

Codecov Report

Merging #1027 (50083e5) into main (e6a4e8f) will decrease coverage by 0.06%.
The diff coverage is 67.56%.

@@            Coverage Diff             @@
##             main    #1027      +/-   ##
==========================================
- Coverage   63.87%   63.81%   -0.07%     
==========================================
  Files         238      239       +1     
  Lines       11179    11211      +32     
==========================================
+ Hits         7141     7154      +13     
- Misses       3206     3221      +15     
- Partials      832      836       +4     

Impacted Files	Coverage Δ
internal/tjson/datetime.go	56.00% <56.00%> (ø)
internal/tjson/tjson.go	70.06% <88.88%> (+1.72%)	⬆️
integration/shareddata/shareddata.go	82.53% <100.00%> (ø)
internal/tjson/schema.go	69.60% <100.00%> (+2.13%)	⬆️
internal/clientconn/conninfo/conn_info.go	66.66% <0.00%> (-16.67%)	⬇️
internal/handlers/pg/msg_dbstats.go	65.11% <0.00%> (-6.98%)	⬇️
internal/util/version/version.go	55.00% <0.00%> (-5.00%)	⬇️
internal/handlers/pg/pgdb/settings.go	39.53% <0.00%> (-4.66%)	⬇️
internal/handlers/common/filter.go	85.38% <0.00%> (ø)
... and 1 more

Flag	Coverage Δ
integration	59.35% <59.45%> (-0.09%)	⬇️
mongodb	18.06% <100.00%> (-0.02%)	⬇️
pg	61.99% <100.00%> (-0.13%)	⬇️
tigris	26.75% <59.45%> (+0.23%)	⬆️
unit	24.25% <62.85%> (-0.49%)	⬇️

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

[AlekSi]

Looks good overall

[w84thesun]

LGTM