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After #214846 (closed) is ready, we need to add the actual GraphQL API and views to be able to add sprints

This includes ONLY group views. Initially we will only create group sprints.

This Issue is only related to Sprint creation and assignment to Issues.

Example GraphQL query for getting group sprints:

{
  group(fullPath: "gitlab-org") {
    sprints(first: 10) {
      nodes {
        title
        state
      }
    }
  }
}

which returns, for example,

{
  "data": {
    "group": {
      "sprints": {
        "nodes": [
          {
            "title": "1",
            "state": "active"
          }
        ]
      }
    }
  }
}