Introduce DataIterator and IDataDriver

[leonard84]

The main motivation is to extract the DataIterator logic from the
PlatformParameterizedSpecRunner into a reusable form, so that it can
serve as foundation for data driven specs in the future.

This also opened up the possibility of adding a new extension point IDataDriver,
which can be used to control the actual iterations of a feature.
A possible use-case is to enable property based testing like jqwik
via a third-party extension.

[codecov]

Codecov Report

Merging #1479 (9261e4a) into master (a897f6b) will increase coverage by 79.73%.
The diff coverage is 83.61%.

@@              Coverage Diff              @@
##             master    #1479       +/-   ##
=============================================
+ Coverage          0   79.73%   +79.73%     
- Complexity        0     4024     +4024     
=============================================
  Files             0      411      +411     
  Lines             0    12710    +12710     
  Branches          0     1649     +1649     
=============================================
+ Hits              0    10134    +10134     
- Misses            0     1981     +1981     
- Partials          0      595      +595     

Impacted Files	Coverage Δ
.../org/spockframework/runtime/model/FeatureInfo.java	95.45% <50.00%> (ø)
...ework/runtime/PlatformParameterizedSpecRunner.java	71.42% <70.00%> (ø)
...rg/spockframework/runtime/DataIteratorFactory.java	80.95% <80.95%> (ø)
...ork/runtime/ParameterizedFeatureChildExecutor.java	95.45% <92.30%> (ø)
...a/org/spockframework/runtime/model/MethodInfo.java	95.16% <92.85%> (ø)
...ockframework/runtime/ParameterizedFeatureNode.java	78.18% <100.00%> (ø)
.../spockframework/runtime/SpockExecutionContext.java	95.34% <100.00%> (ø)
.../spockframework/runtime/extension/IDataDriver.java	100.00% <100.00%> (ø)
.../spockframework/runtime/model/ExecutionResult.java	100.00% <100.00%> (ø)
...rg/spockframework/runtime/model/ParameterInfo.java	100.00% <100.00%> (ø)
... and 407 more

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[wolfs]

I used this to replace an ugly hack for Gradle's multi version extension, and it works good! So I'd like to have this as a feature and would use it.