[JENKINS-72958] expose buildable property to API

[jtnord]

jenkinsci/branch-api-plugin#468 but on the base class that defines isBuildable

See JENKINS-73425.

Obsoletes jenkinsci/branch-api-plugin#468 (untested)

Proposed changelog entries

• Expose buildable for ComputedFolders to the REST API

Submitter checklist

• JIRA issue is well described
• Changelog entry appropriate for the audience affected by the change (users or developer, depending on the change).
  * Use the Internal: prefix if the change has no user-visible impact (API, test frameworks, etc.)
• Appropriate autotests or explanation to why this change has no tests

[jglick]

(pending testing as per jenkinsci/branch-api-plugin#468 (comment))

[MarkRx]

@jtnord tested works thanks!

http://localhost:8080/jenkins/job/folder1/job/pipeline1/api/json?tree=buildable

Enabled:

{
    _class: "org.jenkinsci.plugins.workflow.multibranch.WorkflowMultiBranchProject",
    buildable: true
}

Disabled:

{
    _class: "org.jenkinsci.plugins.workflow.multibranch.WorkflowMultiBranchProject",
    buildable: false
}

Note child jobs will have buildable=true even if the parent is disabled (buildable=false). The buildable property doesn't propagate so the parent job needs to be checked. This is existing behavior and unrelated to exposing the buildable property.