gatsby-plugin-google-tagmanager: accept arrays for defaultDataLayer

[mfanuzzi]

Let's say you need to set up Google Consent Mode. That requires two objects to be pushed to the top of the dataLayer. With gatsby-plugin-google-tagmanager, I expected that I'd be able to pass an array for the defaultDataLayer option like so:

    defaultDataLayer: 
      [
        {
          consent: {
            default: {
              ad_storage: 'denied',
              analytics_storage: 'denied',
              wait_for_update: 500,
            },
          },
        },
        {
          set: {
            ads_data_redaction: true,
          },
        },
      ],

But this is not supported, it looks like because it wants to just push onto dataLayer the entirety of defaultDataLayer and be done. Reasonable, but it would not be hard to, for example, check if defaultDataLayer is an array, and if so, add them in a loop (or merge/spread, whatever). The same way it conditionally checks if it's a function and executes it if so.

Here is my current hack, exploiting the ability for the plugin to call a function:

    defaultDataLayer: () => {
      [
        {
          consent: {
            default: {
              ad_storage: 'denied',
              analytics_storage: 'denied',
              wait_for_update: 500,
            },
          },
        },
        {
          set: {
            ads_data_redaction: true,
          },
        },
      ].forEach((d) => window['dataLayer'] && window['dataLayer'].push(d)); // <- this could be done in the plugin
      return {
        platform: 'gatsby',
      } // <- this is just to return something from the function so it doesn't complain;
    },

This works, but it would of course be better and clearer to just be able to pass in array in the first place.

If I or my team get time, we may submit a PR for this if there's interest.

(Side note on that: forking a huge monorepo to make a small contribution kinda stinks; any plans on moving away from the monolith?)