WorkItem: Add GraphQl mutation to update color

What does this MR do and why?

Add mutation to update color

GraphQl mutation to update color widget of a work item

Issue: #394863 (closed)

MR acceptance checklist

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GraphQl Query

mutation {
  workItemUpdate(
    input: {id: "gid://gitlab/WorkItem/652", colorWidget: {color: "#348333"}}
  ) {
    workItem {
      ...WorkItem
      __typename
    }
    errors
    __typename
  }
}

fragment WorkItem on WorkItem {
  id
  iid
  title
  widgets {
    ...WorkItemWidgets
    __typename
  }
  __typename
}

fragment WorkItemWidgets on WorkItemWidget {
  ... on WorkItemWidgetColor {
    type
    color
    textColor
    __typename
  }
}

GraphQl Response

{
  "data": {
    "workItemUpdate": {
      "workItem": {
        "id": "gid://gitlab/WorkItem/652",
        "iid": "1",
        "title": "TEsting",
        "widgets": [
          {
            "__typename": "WorkItemWidgetAssignees"
          },
          {
            "__typename": "WorkItemWidgetDescription"
          },
          {
            "__typename": "WorkItemWidgetHierarchy"
          },
          {
            "__typename": "WorkItemWidgetLabels"
          },
          {
            "__typename": "WorkItemWidgetNotes"
          },
          {
            "__typename": "WorkItemWidgetStartAndDueDate"
          },
          {
            "__typename": "WorkItemWidgetHealthStatus"
          },
          {
            "__typename": "WorkItemWidgetStatus"
          },
          {
            "__typename": "WorkItemWidgetNotifications"
          },
          {
            "__typename": "WorkItemWidgetAwardEmoji"
          },
          {
            "__typename": "WorkItemWidgetLinkedItems"
          },
          {
            "__typename": "WorkItemWidgetCurrentUserTodos"
          },
          {
            "type": "COLOR",
            "color": "#348333",
            "textColor": "#FFFFFF",
            "__typename": "WorkItemWidgetColor"
          }
        ],
        "__typename": "WorkItem"
      },
      "errors": [],
      "__typename": "WorkItemUpdatePayload"
    }
  }
}

How to set up and validate locally

2. Enable namespace_level_work_items FF - Feature.enable(:namespace_level_work_items)
3. Once done, visit any group on GDK and then navigate to its /work_items path, eg; https://gitlab.com/groups/gitlab-org/plan-stage/-/work_items. You should see a Create epic button and that will create a Work Item at group-level with type Epic. Copy the Gid of it.
4. Got to graphql-explorer and enter the mutation mentioned above with the newly created epic Id. You should see color widget updated value.

Related to #394863 (closed)