[gatsby-plugin-google-gtag] Accept a measurementIds in options

[tcjcodes]

Setting up a brand new web stream in the analytics gtag setup gives a "Measurement ID". It seems as though this replaces the trackingId that was accepted before (based on #27627).

This seems to still work with the plugin, since it calls gtag('config', trackingId) here, but the field name should probably be updated to match.

Here's the recommended code from google from the Add new on-page tag section:

<!-- Global site tag (gtag.js) - Google Analytics -->
<script async src="https://www.googletagmanager.com/gtag/js?id=MEASUREMENT_ID"></script>
<script>
  window.dataLayer = window.dataLayer || [];
  function gtag(){dataLayer.push(arguments);}
  gtag('js', new Date());

  gtag('config', /*MEASUREMENT_ID*/);
</script>