Rework step conditional execution

[ccremer]

Summary

Predicates wrapping steps doesn't quite feel right. For instance, previous If implementation created an additional step that had the same name as for the step that would be executed if it evaluated to true.

This PR makes conditional and predicates first-class citizen in the pipeline. Step has now a new field Condition Predicate[T]. If set, the pipeline will evaluate the condition prior to executing a Step's Action().

Change summary:

• pipeline.If is removed. use NewStep(...).When(predicate) instead.
• pipeline.IfOrElse is removed, use NewStep(...).When(pipeline.Not(predicate) instead.
• New method Pipeline.When(predicate, name, action
• New func NewStepIf(predicate, name, action)
• New method Step.When(predicate)
• Pipeline.WithNestedSteps has now Predicate[T] in the signature. It accepts nil as well.

Checklist

• Categorize the PR by setting a good title and adding one of the labels:
  fix, enhancement, documentation, change, breaking, dependency
  as they show up in the changelog
• Update documentation.
• Update tests.
• Link this PR to related issues.