Add certs health check endpoint (LG-3929)

[zachmargolis]

Example curl:

curl -v -k "https://localhost:8443/api/health/certs" --silent | jq
....
< HTTP/1.1 500 Internal Server Error
....
{
  "healthy": false,
  "info": {
    "deadline": "2021-01-15T18:07:58.244Z",
    "expiring": [
      {
        "expiration": "2020-12-15T21:10:27.000Z",
        "subject": "/C=US/O=Entrust/OU=Certification Authorities/OU=Entrust Managed Services Root CA",
        "issuer": "/C=US/O=U.S. Government/OU=FPKI/CN=Federal Common Policy CA",
        "key_id": "9C:62:66:26:9D:71:B6:A7:75:53:64:E1:AC:B1:C7:25:3C:44:5D:0D"
      },
      {
        "expiration": "2021-01-12T00:52:59.000Z",
        "subject": "/C=US/O=ORC PKI/CN=ORC SSP 3",
        "issuer": "/C=US/O=U.S. Government/OU=FPKI/CN=Federal Common Policy CA",
        "key_id": "6A:88:6D:52:FC:B1:44:9E:30:AE:33:18:4D:C0:39:9D:96:6B:24:B0"
      }
    ]
  }
}

[mitchellhenke]

👍🏼 👍🏼