[BE] Implement create restaurant group mutation

[shahanneda]

Implement a graphql mutation for creating a resturant group. You should also be making a new service as part of this change.

Here is a sample of how it should look:

mutation {
    createRestaurantGroup(name: "Open Late", description: "Late Night Snack Restaurants", restaurantIds: [645b05286f45f7dcaac617b7, 645b05296f45f7dcaac617b8, 645b05296f45f7dcaac617b9]) {
        id
        name
        description
        restaurants
    }
}