Add pending health status

[dhurley]

Proposed changes

Example:

{
  "status": "PENDING",
  "checks": [
    {
      "name": "registration",
      "status": "OK"
    },
    {
      "name": "commandConnection",
      "status": "OK"
    },
    {
      "name": "metricsConnection",
      "status": "PENDING"
    }
  ]
}

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• [ ] I have read the CONTRIBUTING document
• [ ] I have run make install-tools and have attached any dependency changes to this pull request
• [ ] If applicable, I have added tests that prove my fix is effective or that my feature works
• [ ] If applicable, I have checked that any relevant tests pass after adding my changes
• [ ] If applicable, I have updated any relevant documentation (README.md)
• [ ] If applicable, I have tested my cross-platform changes on Ubuntu 22, Redhat 8, SUSE 15 and FreeBSD 13

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