added list buckets query

internal/gql/schema.go

@@ -100,6 +100,21 @@ func GraphqlSchema() (graphql.Schema, error) {
 				return auth, nil
 			},
 		},
+		"buckets": &graphql.Field{
+			Name:        "buckets",
+			Type:        graphql.NewNonNull(graphql.NewList(graphql.String)),
+			Description: "List available buckets",
+			Resolve: func(p graphql.ResolveParams) (interface{}, error) {
+				if !helper.IsAuthenticated(p.Context) {
+					return nil, s3errors.ErrNotAuthenticated
+				}
+
+				loader := p.Context.Value("loader").(map[string]*dataloader.Loader)
+				// The only reason we use a dataloader with a empty key is that we want to cache the result
+				thunk := loader["listBuckets"].Load(p.Context, dataloader.StringKey(""))
+				return thunk()
+			},
+		},
 	}
 
 	mutationFields := graphql.Fields{