feat: EvaluationContext add constructor without TargetingKey

[thomaspoignant]

This PR

Based on this discussion we have discovered that in some cases we want to create an EvaluationContext without Targeting key.

WithNewEvaluationContext it was mandatory to set a targetingKey forcing us to set an empty targetingKey sometimes.

In this PR we add a new constructor called NewTargetlessEvaluationContext that is doing exactly the same things as NewEvaluationContext but does not force you to add a targetingKey.

[codecov[bot]]

Codecov Report

Merging #204 (40ffb01) into main (a2987b8) will increase coverage by 0.03%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #204      +/-   ##
==========================================
+ Coverage   78.95%   78.99%   +0.03%     
==========================================
  Files           9        9              
  Lines        1074     1076       +2     
==========================================
+ Hits          848      850       +2     
  Misses        204      204              
  Partials       22       22              

Files Changed	Coverage Δ
pkg/openfeature/evaluation_context.go	82.60% <100.00%> (+1.65%)	:arrow_up:

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