[DOP-24139] Add method for creating namespaces

[IlyasDevelopment]

Change Summary

• Added method for creating namespaces
• Fixed fixture with namespace existence check

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 85.71429% with 3 lines in your changes missing coverage. Please review.

Project coverage is 92.02%. Comparing base (db5181f) to head (3b895fe).
Report is 12 commits behind head on develop.

Files with missing lines	Patch %	Lines
horizon_hwm_store/horizon_hwm_store.py	82.35%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #44      +/-   ##
===========================================
- Coverage    92.18%   92.02%   -0.16%     
===========================================
  Files            4        4              
  Lines          128      138      +10     
  Branches         9       10       +1     
===========================================
+ Hits           118      127       +9     
  Misses           7        7              
- Partials         3        4       +1     

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