Add draft ItemConfigurator support for WorkflowJob

[somiljain2006]

See jenkinsci/configuration-as-code-plugin#2881

Add a draft PipelineItemConfigurator implementation demonstrating how the proposed ItemConfigurator extension point can be used by the Workflow Job plugin to create and configure WorkflowJob instances through JCasC. This proof of concept supports configuring a job's name, description, and an inline CPS pipeline definition (CpsFlowDefinition), illustrating that the API can be extended beyond freestyle jobs. Example configuration:

items:
  - pipeline:
      name: "my-ci-pipeline"
      description: "Builds the project"
      definition:
        cps:
          script: |
            node {
              echo 'Hello Pipeline!'
            }

Testing done

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