vars plugin: allow to control staging and caching

[felixfontein]

Staging allows to control when a vars plugin runs, see https://github.com/ansible/ansible/pull/61078/files for details.

Allowing to disable caching allows to modify vars files during the playbook, with the changes being available in the next run.

[codecov]

Codecov Report

Merging #32 into main will decrease coverage by 0.21%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##             main      #32      +/-   ##
==========================================
- Coverage   86.51%   86.29%   -0.22%     
==========================================
  Files           6        6              
  Lines         341      343       +2     
  Branches       60       61       +1     
==========================================
+ Hits          295      296       +1     
  Misses         32       32              
- Partials       14       15       +1     

Impacted Files	Coverage Δ
plugins/vars/sops.py	87.71% <66.66%> (-1.38%)	⬇️

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[felixfontein]

Restarting tests since ansible-test sanity should no longer fail for stable-2.9.

[felixfontein]

@endorama thanks for reviewing this!