Add draft ItemConfigurator support for Folder

[somiljain2006]

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

Add a draft FolderItemConfigurator demonstrating how the proposed ItemConfigurator extension point can be implemented by the CloudBees Folder plugin to create and configure Folder instances through JCasC. As a proof of concept, this implementation supports configuring a folder's name, display name, and description, illustrating that the API can be extended beyond freestyle and pipeline jobs.

Example configuration:

items:
  - folder:
      name: "team-alpha-jobs"
      displayName: "Team Alpha"
      description: "All CI/CD jobs for Team Alpha"

Proposed changelog entries

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• JIRA issue is well described
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[timja]

I assume this doesn't work with credentials as yet? Credentials are the biggest pain point I'm aware of with folders and job dsl

[somiljain2006]

Not yet. At the moment, this configurator only supports creating/updating the folder itself, along with description and displayName. I haven't added support for configuring folder credentials yet.

[somiljain2006]

I implemented folder-scoped credentials support and verified that it works. However, adding cloudbees-folder and credentials as compile-time dependencies to JCasC core introduces architectural problems because those plugins already integrate with JCasC. Rather than introducing reflection or other workarounds, I think the correct home for the folder-specific configurators is the cloudbees-folder plugin itself.

[timja]

yes all plugin specific configurator's should be in the plugins themselves.

JCasC only provides direct implementations for Jenkins core features