Add validationStartedAt to DastSiteProfileType

What does this MR do and why?

This Merge Request adds the validation_started_at to the DastSiteProfileType to allow the frontend to verify when a validation started as discussed here.

This Merge Request is related to issue #369938.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Go to the /-/graphql-explorer

2. Create a query like

{
  project(fullPath: "root/apiscantest") {
    dastSiteProfiles {
      edges {
        node {
          validationStatus
          validationStartedAt
        }
      }
    }
  }
}

Check the result

{
  "data": {
    "project": {
      "dastSiteProfiles": {
        "edges": [
          {
            "node": {
              "validationStatus": "FAILED_VALIDATION",
              "validationStartedAt": "2022-10-21T17:47:21Z"
            }
          }
        ]
      }
    }
  }
}

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.