Add function to return date of most recent UKWA capture of a URL

[anjackson]

Our curators have suggested that a function that returns the most recent crawl date (rather than the crawl status) would be useful. e.g.

=WEBARCHIVE_LAST_MEMENTO_DATE_UKWA({url})

[machawk1]

  const headers = response.getHeaders()
  if ('memento-datetime' in headers) {
    return headers['memento-datetime']
  } else {
      return ""
  }

...as the final line of the new function should accomplish this. It could also be pared down to a more succinct, less expressive form. Also, you may need to adapt to Memento-Datetime capitalization per-archive. UKWA appears to use all-lowercase and JS objects are case sensitive.