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')
```

Start by reviewing the Java client API's existing job and folder object models and the Multibranch workaround shown in the issue. Define the required Multibranch models and retrieval behavior, then verify that Multibranch projects can be used directly without mimicking them as folders.

Escrito por el modelo de indexación a partir del texto del issue.