Allow choosing OPA evaluator

[joejstuart]

This just creates the option to use OPA by setting the EC_USE_OPA env variable.

https://issues.redhat.com/browse/EC-1044

[codecov]

Codecov Report

Attention: Patch coverage is 84.21053% with 3 lines in your changes missing coverage. Please review.

Project coverage is 72.72%. Comparing base (9d4f0a8) to head (9eb4a60).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/evaluator/opa_evaluator.go	80.00%	2 Missing ⚠️
cmd/validate/image.go	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2252      +/-   ##
==========================================
+ Coverage   72.69%   72.72%   +0.02%     
==========================================
  Files          88       89       +1     
  Lines        7622     7640      +18     
==========================================
+ Hits         5541     5556      +15     
- Misses       2081     2084       +3     

Flag	Coverage Δ
generative	72.72% <84.21%> (+0.02%)	⬆️
integration	72.72% <84.21%> (+0.02%)	⬆️
unit	72.72% <84.21%> (+0.02%)	⬆️

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Files with missing lines	Coverage Δ
internal/utils/helpers.go	70.73% <100.00%> (+0.73%)	⬆️
cmd/validate/image.go	90.50% <85.71%> (-0.10%)	⬇️
internal/evaluator/opa_evaluator.go	80.00% <80.00%> (ø)