[DOP-15650] Rename permissions endpoint

[dolfinus]

Change Summary

Namespace related endpoints look like /v1/namespaces/:id. But endpoint for permissions look like /v1/namespace/:id/permissions. Rename it to keep endpoints consistent.

This is a breaking change, so bumping version to 0.2.x.

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 96.55172% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 91.47%. Comparing base (4313940) to head (f69d6e3).

Files	Patch %	Lines
horizon/backend/api/v1/router/namespaces.py	96.42%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #61      +/-   ##
===========================================
- Coverage    91.50%   91.47%   -0.04%     
===========================================
  Files          120      119       -1     
  Lines         2449     2440       -9     
  Branches       240      240              
===========================================
- Hits          2241     2232       -9     
  Misses         162      162              
  Partials        46       46              

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