Hello, it would be great if we had a mechanism to define dynamic stages within declarative pipelines. The use case is that I want to create a pipeline function for micro service repositories. Each repository contains a number of container images that needs to be built in parallel. The only way I found is using the these pseudo stages wrapped by the `parallel` step:

pipeline {
  agent any
  stages {
    stage('build') {
      steps {
        runParallel items: ("a".."f").collect { "Stage ${it}" }
      }
    }
  }
}

def runParallel(args) {
  parallel args.items.collectEntries { name -> [ "${name}": {
    stage("${name}") {
      echo name
    }
  }]}
}

This is bad because the sub-stages can neither have an agent nor contain other sub-stages. And it requires a separate function (I couldn't get it working without).
On the other hand, this shouldn't make the declarative pipeline too scripty. A minimal scriptiness approach would look like this:

pipeline {
  agent none
  stages {
    stage('build') {
      stages {
        ("A".."F").collect {
          stage("Stage ${it}") {
            agent any
            steps {
              echo "Hello from stage ${it}!"
            }
          }
        }
      }
    }
  }
}