JENKINS-45898 Proper Multibranch pipeline support

Currently, the Java client api does not have proper support for Multibranch projects. Since these differ a bit from other types of jobs, how to use the Java client api's are not intuitive. The code base does not really take Multibranch projects in to account either. It would preferable if these could be supported out of the box with proper object models.

The current workaround is to mimic the Multibranch job as a folder, such as:
```
FolderJob multibranchRoot = new FolderJob(createdJob.getName(), jenkinsUrl + '/job/' + createdJob.getName() + '/')
def job = jenkins.getJob(multibranchRoot, 'master')
```

まず、Java API に既に存在する job と folder のオブジェクトモデル、および issue に示されている Multibranch の workaround を確認します。必要な Multibranch モデルと取得動作を定義し、そのうえで、Multibranch プロジェクトを folder として模倣せずに直接使用できることを検証します。

索引モデルが issue の本文から書いたものです。