Add support for grouped legends

[clementprdhomme]

This PR adds support for grouped legends by reusing the “basic” legend type which already had most of the feature implemented.

To defined groups, nest an items array inside each item. You can test the implementation by defining a legend as follows:

{
  "type": "basic",
  "items": [
    {
      "name": "Drivers of temporary disturbances",
      "items": [
        {
          "name": "Forest management",
          "color": "#FF00DD"
        },
        {
          "name": "Shifting cultivation",
          "color": "#34EE64"
        },
        {
          "name": "Wildfires",
          "color": "#A200FF"
        },
        {
          "name": "Other natural disturbances",
          "color": "#DDD"
        },
      ]
    },
    {
      "name": "Drivers of deforestation",
      "items": [
        {
          "name": "Hard commodities",
          "color": "#FFDEDD"
        },
        {
          "name": "Permanent agriculture",
          "color": "#D4E36D"
        },
        {
          "name": "Settlements and infrastructure",
          "color": "#0200FF"
        }
      ]
    }
  ]
}

Tracking

FLAG-1231