[Admin] Create and update tax rates

[chaimann]

[!IMPORTANT] Blocked by #6294

Summary

Adds interface to create and update Tax Rates

Checklist

Check out our PR guidelines for more details.

The following are mandatory for all PRs:

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

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 86.30137% with 10 lines in your changes missing coverage. Please review.

Project coverage is 92.91%. Comparing base (bbca5cc) to head (151b817).

Files with missing lines	Patch %	Lines
admin/app/lib/solidus_admin/form_builder.rb	65.51%	10 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6298      +/-   ##
==========================================
+ Coverage   86.65%   92.91%   +6.26%     
==========================================
  Files         521      417     -104     
  Lines       11943     8342    -3601     
==========================================
- Hits        10349     7751    -2598     
+ Misses       1594      591    -1003     

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