Admin payment methods crud

[chaimann]

[!IMPORTANT] blocked by #6294

Summary

Checklist

Check out our PR guidelines for more details.

The following are mandatory for all PRs:

• [ ] I agree that my PR will be published under the same license as Solidus.
• [ ] I have written a thorough PR description.
• [ ] I have kept my commits small and atomic.
• [ ] I have used clear, explanatory commit messages.

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• 📖 I have updated the README to account for my changes.
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[codecov[bot]]

Codecov Report

:x: Patch coverage is 89.70588% with 7 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 88.97%. Comparing base (bbca5cc) to head (e701f96). :warning: Report is 123 commits behind head on main.

Files with missing lines	Patch %	Lines
admin/app/lib/solidus_admin/form_builder.rb	79.41%	7 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6302      +/-   ##
==========================================
+ Coverage   86.65%   88.97%   +2.31%     
==========================================
  Files         521      867     +346     
  Lines       11943    18471    +6528     
==========================================
+ Hits        10349    16434    +6085     
- Misses       1594     2037     +443     

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