feat: add starts_with and ends_with json evaluators

[bacherfl]

This PR adds additional jsonLogic evaluators for starts_with and ends_with, which check if the given property starts/ends with the given value. Both evaluators return a boolean, indicating whether the condition has been met.

The following example shows how these evaluators can be used in a flag targeting rule:

{
  "flags": {
    "headerColor": {
      "state": "ENABLED",
      "defaultVariant": "red",
      "variants": {
        "blue": "#0000FF",
        "green": "#00FF00",
        "red": "#FF0000",
        "yellow": "#FFFF00"
      },
      "targeting": {
        "if": [
          {
            "starts_with": [
              {
                "var": "email"
              },
              "user@faas"
            ]
          },
          "red",
          "green"
        ]
      },
      "source": ""
    }
  }
}

This rule will resolve to the red variant of the headerColor flag, if the email property within the evaluationContext starts with user@faas.

[codecov]

Codecov Report

Merging #658 (937e5e9) into main (07407a3) will increase coverage by 0.40%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #658      +/-   ##
==========================================
+ Coverage   72.98%   73.39%   +0.40%     
==========================================
  Files          24       25       +1     
  Lines        2402     2439      +37     
==========================================
+ Hits         1753     1790      +37     
  Misses        583      583              
  Partials       66       66              

Impacted Files	Coverage Δ
core/pkg/eval/json_evaluator.go	89.53% <100.00%> (+0.24%)	⬆️
core/pkg/eval/string_comparison_evaluation.go	100.00% <100.00%> (ø)

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[beeme1mr]

The functionality looks good, but it needs to be documented.

[bacherfl]

The functionality looks good, but it needs to be documented.

the documentation has now been added as well