Add group as an omittable field

[Elchi3]

Illustration for #648

[foolip]

I'd like to treats groups like specs and make it a string or list or strings depending on how many there are. I think a single group is going to be common, probably more common than multiple groups. So I'd call it group in singular just like spec.

[Elchi3]

Updated to be consistent with the spec field. Like for spec, when there is just one entry, I've removed the yaml list. So, If this sees adaption in the schema, we will need something like this:

"group": {
  "anyOf": [
    {
      "description": "Group identifier",
      "type": "string"
    },
    {
      "items": {
        "description": "Group identifier",
        "type": "string"
      },
      "minItems": 2,
      "type": "array"
    }
  ],
  "description": "Group"
}, 

[foolip]

Since it's not published and doesn't get in the way, I'd be happy to merge this. We're still discussing how to represent group properties (name/description/etc.) and group hierarchy in #648, but we can do that in a separate PR.

[foolip]

@Elchi3 I haven't merged this because it's draft, but I see your 👍 and will undraft it myself for merging :)