Adds crossProjectPipelineAvailable to NamespaceType

What does this MR do and why?

Add crossProjectPipelineAvailable to NamespaceType. This is needed in order to complete the frontend work: #348303 (closed), which prevents the multi-project pipeline graph being available on GitLab Free

BE issue: #350809 (closed)

How to set up and validate locally

query:

{
  project(fullPath: "root/the-lodge") {
    pipeline(iid: "17") {
      user {
        namespace{
      	  crossProjectPipelineAvailable
        }
      }
    }
  }
}

response:

{
  "data": {
    "project": {
      "pipeline": {
        "user": {
          "namespace": {
            "crossProjectPipelineAvailable": false
          }
        }
      }
    }
  }
}

MR acceptance checklist

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