Added lazy-in-viewport custom metric

[kevinfarrugia]

Updated the Images custom metric to include two additional properties, loading - which retrieves the value of the loading attribute on the <img> element and in-viewport - which returns a boolean value indicating whether the image is visible in the viewport.

Will be used in 2021 Resource Hints chapter.

Source: https://docs.webpagetest.org/custom-metrics/examples/lazy-images-in-viewport/

Sample WPT https://webpagetest.org/custom_metrics.php?test=210602_BiDcM8_940d25ae2138a2d2dae32c111fb964cc&run=1&cached=0

[
  {
    "url": "https://placekitten.com/401/401",
    "width": 401,
    "height": 401,
    "naturalWidth": 401,
    "naturalHeight": 401,
    "loading": "lazy",
    "in-viewport": true
  },
  {
    "url": "https://placekitten.com/402/402",
    "width": 402,
    "height": 402,
    "naturalWidth": 402,
    "naturalHeight": 402,
    "loading": "lazy",
    "in-viewport": false
  }
]