feat: default variant redo

[markphelps]

Redo of #3168

Re: #3134

TODO

• Add tests
• Implement in UI
• Add for other relational dbs
• Think about/implement in non-relational dbs
• Add to API docs
• Add to client-side eval logic
• Update OpenFeature providers

[codecov]

Codecov Report

Attention: Patch coverage is 51.28205% with 19 lines in your changes missing coverage. Please review.

Project coverage is 64.05%. Comparing base (3e72b57) to head (9e88dc5).

Files	Patch %	Lines
internal/server/evaluation/legacy_evaluator.go	7.69%	8 Missing and 4 partials ⚠️
internal/storage/sql/common/flag.go	73.07%	3 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3271      +/-   ##
==========================================
- Coverage   64.09%   64.05%   -0.05%     
==========================================
  Files         168      168              
  Lines       13393    13417      +24     
==========================================
+ Hits         8584     8594      +10     
- Misses       4149     4158       +9     
- Partials      660      665       +5     

Flag	Coverage Δ
unittests	64.05% <51.28%> (-0.05%)	⬇️

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