Added version 9 of API and new worker metrics

[oliveromahony]

Proposed changes

Added support for NGINX Plus API version 9

Added new workers metrics /api/9/workers

[
  {
    "id": 0,
    "pid": 13,
    "connections": {
      "accepted": 81,
      "dropped": 0,
      "active": 1,
      "idle": 0
    },
    "http": {
      "requests": {
        "total": 100,
        "current": 1
      }
    }
  },
  {
    "id": 1,
    "pid": 8,
    "connections": {
      "accepted": 81,
      "dropped": 0,
      "active": 0,
      "idle": 0
    },
    "http": {
      "requests": {
        "total": 201,
        "current": 0
      }
    }
  },
  {
    "id": 2,
    "pid": 9,
    "connections": {
      "accepted": 82,
      "dropped": 0,
      "active": 0,
      "idle": 0
    },
    "http": {
      "requests": {
        "total": 119,
        "current": 0
      }
    }
  },
  {
    "id": 3,
    "pid": 10,
    "connections": {
      "accepted": 82,
      "dropped": 0,
      "active": 0,
      "idle": 0
    },
    "http": {
      "requests": {
        "total": 143,
        "current": 0
      }
    }
  },
  {
    "id": 4,
    "pid": 11,
    "connections": {
      "accepted": 81,
      "dropped": 0,
      "active": 0,
      "idle": 0
    },
    "http": {
      "requests": {
        "total": 156,
        "current": 0
      }
    }
  }
]

Checklist

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• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork