[DOP-13337] Add Pydantic v2 support

[dolfinus]

Change Summary

Allow using horizon-hwm-store with both Pydantic v1 and v2 (thanks to etl-entities 2.3 release). Because Horizon client is inherited from pydantic.BaseModel and not pydantic.v1.BaseModel, this introduced few quirks to validation.

Added tests for both Pydantic versions to CI workflow.

Related issue number

Checklist

• Commit message and PR title is comprehensive
• Keep the change as small as possible
• Unit and integration tests for the changes exist
• Tests pass on CI and coverage does not decrease
• Documentation reflects the changes where applicable
• docs/changelog/next_release/<pull request or issue id>.<change type>.rst file added describing change
  (see CONTRIBUTING.rst for details.)
• My PR is ready to review.

[codecov]

Codecov Report

Attention: Patch coverage is 88.57143% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 93.51%. Comparing base (e29a0c1) to head (ef574e7).

Files	Patch %	Lines
horizon_hwm_store/horizon_hwm_store.py	88.23%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #14      +/-   ##
===========================================
- Coverage    96.62%   93.51%   -3.12%     
===========================================
  Files            3        3              
  Lines           89      108      +19     
  Branches         8       16       +8     
===========================================
+ Hits            86      101      +15     
- Misses           3        5       +2     
- Partials         0        2       +2     

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