Make _id field required in data documents

[noisersup]

Description

This PR closes #1150.

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.
• I ensured that the title is good enough for the changelog.
• (for maintainers only) I set Reviewers (@FerretDB/core), Assignee, Labels, Project and project's Sprint fields.
• I marked all done items in this checklist.

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Name	Status	Preview	Updated
ferret-db	✅ Ready (Inspect)	Visit Preview	Oct 21, 2022 at 10:27AM (UTC)

[codecov]

Codecov Report

Merging #1278 (4cc2f4b) into main (ea39dff) will increase coverage by 20.34%.
The diff coverage is 100.00%.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1278       +/-   ##
===========================================
+ Coverage   46.93%   67.28%   +20.34%     
===========================================
  Files         275      275               
  Lines       12857    12867       +10     
===========================================
+ Hits         6035     8658     +2623     
+ Misses       6095     3302     -2793     
- Partials      727      907      +180     

Impacted Files	Coverage Δ
internal/types/document_validation.go	92.30% <100.00%> (+4.80%)	⬆️
internal/util/version/version.go	48.52% <0.00%> (-4.42%)	⬇️
internal/clientconn/conn.go	44.01% <0.00%> (+0.70%)	⬆️
integration/helpers.go	87.23% <0.00%> (+1.06%)	⬆️
internal/handlers/pg/pgdb/collections.go	50.00% <0.00%> (+2.27%)	⬆️
internal/handlers/pg/pgdb/settings.go	39.53% <0.00%> (+2.32%)	⬆️
internal/handlers/pg/pgdb/tables.go	64.70% <0.00%> (+2.94%)	⬆️
internal/handlers/pg/pgdb/databases.go	55.00% <0.00%> (+5.00%)	⬆️
internal/handlers/common/limit.go	75.00% <0.00%> (+8.33%)	⬆️
... and 50 more

Flag	Coverage Δ
integration	62.81% <77.77%> (+25.49%)	⬆️
mongodb	14.15% <0.00%> (-0.04%)	⬇️
pg	52.33% <77.77%> (?)
tigris	34.71% <77.77%> (+<0.01%)	⬆️
unit	25.69% <100.00%> (+0.03%)	⬆️

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

[rumyantseva]

LGTM

[rumyantseva]

The code looks good to me, but I have one idea to discuss with everyone :)

[w84thesun]

LGTM

[rumyantseva]

LGTM