[Uptime] Add certificate expiration dates to uptime GQL schema - #43352

Summary

The purpose of this patch is to add TLS certificate expiration date/time to our GraphQL schema.

We can now get this information in a format like:

{
  "state": {
    "monitor": {
      "name": "elastic-website"
    },
    "tls": {
      "certificate_not_valid_after": [
        "2020-07-16T03:15:39.000Z"
      ]
    }
  }
}

Testing this PR

0. Set up Heartbeat/Kibana/ES (ping me if further instruction needed)
1. Configure a monitor for a TLS-enabled website (you can use Elastic's website):

- type: http
  schedule: '@every 10s'
  urls: ['https://www.elastic.co/']
  name: 'elastic-website'

2. Use GraphiQL or some other GQL querying tool to query our endpoint like:

{
  getMonitorStates(dateRangeStart: "now-15m", dateRangeEnd: "now") {
    summaries {
      state {
        monitor {
          name
        }
        tls {
          certificate_not_valid_after
        }
      }
    }
  }
}