api: /v1/jobs always include namespaces

[notnoop]

Add Namespace as a top-level field in /v1/jobs stub.

The /v1/jobs endpoint already includes the namespace under JobSummary, though the API is odd, as typically the job ID and Namespace are in the same level, and the oddity complicates the UI frontend development.

The downside of adding it is redundant field, that makes the response body a bit bigger, specially for clusters with large jobs. Though, it should compress nicely and I expect the overhead to be small to overall response size. The benefit of a cleaner and more consistent API seem worth it.

Sample /v1/api response before

[
  {
    "CreateIndex": 10,
    "Datacenters": [
      "dc1"
    ],
    "ID": "example",
    "JobModifyIndex": 10,
    "JobSummary": {
      "Children": {
        "Dead": 0,
        "Pending": 0,
        "Running": 0
      },
      "CreateIndex": 10,
      "JobID": "example",
      "ModifyIndex": 14,
      "Namespace": "default",
      "Summary": {
        "cache": {
          "Complete": 0,
          "Failed": 0,
          "Lost": 0,
          "Queued": 0,
          "Running": 1,
          "Starting": 0
        }
      }
    },
    "ModifyIndex": 11,
    "Multiregion": null,
    "Name": "example",
    "ParameterizedJob": false,
    "ParentID": "",
    "Periodic": false,
    "Priority": 50,
    "Status": "running",
    "StatusDescription": "",
    "Stop": false,
    "SubmitTime": 1619192400425852000,
    "Type": "service"
  }
]

Sample /v1/api response: After

[
  {
    "CreateIndex": 10,
    "Datacenters": [
      "dc1"
    ],
    "ID": "example",
    "JobModifyIndex": 10,
    "JobSummary": {
      "Children": {
        "Dead": 0,
        "Pending": 0,
        "Running": 0
      },
      "CreateIndex": 10,
      "JobID": "example",
      "ModifyIndex": 14,
      "Namespace": "default",
      "Summary": {
        "cache": {
          "Complete": 0,
          "Failed": 0,
          "Lost": 0,
          "Queued": 0,
          "Running": 1,
          "Starting": 0
        }
      }
    },
    "ModifyIndex": 11,
    "Multiregion": null,
    "Name": "example",
    "Namespace": "default",
    "ParameterizedJob": false,
    "ParentID": "",
    "Periodic": false,
    "Priority": 50,
    "Status": "running",
    "StatusDescription": "",
    "Stop": false,
    "SubmitTime": 1619192552309353000,
    "Type": "service"
  }
]

Fixes #10431

[DingoEatingFuzz]

LGTM, thank you for the quick turnaround.

[github-actions]

I'm going to lock this pull request because it has been closed for 120 days ⏳. This helps our maintainers find and focus on the active contributions.
If you have found a problem that seems related to this change, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.