Add reponse helper function for group permissions

Reponse to extract groups and their permissions was added.

Additional definitons for the avilable Bitbucket Server permission
levels were added to allow for easier usage.

Tests were added and udpated.

api_response_test.go

@@ -360,7 +360,7 @@ func TestGetUsersPermissionResponse(t *testing.T) {
 						Slug:        "jcitizen",
 						Type:        "NORMAL",
 					},
-					Permission: "REPO_ADMIN",
+					Permission: PermissionRepositoryAdmin.String(),
 				},
 			},
 			wantErr: false,
@@ -380,6 +380,75 @@ func TestGetUsersPermissionResponse(t *testing.T) {
 	}
 }
 
+func TestGetGroupsPermissionResponse(t *testing.T) {
+	type args struct {
+		r *APIResponse
+	}
+	tests := []struct {
+		name    string
+		args    args
+		want    []GroupPermission
+		wantErr bool
+	}{
+		{
+			name: "Empty list",
+			args: args{
+				r: &APIResponse{
+					Values: map[string]interface{}{"values": []interface{}{}},
+				},
+			},
+			want:    []GroupPermission{},
+			wantErr: false,
+		},
+		{
+			name: "Bad response",
+			args: args{
+				r: &APIResponse{
+					Values: map[string]interface{}{"values": "not an array"},
+				},
+			},
+			want:    nil,
+			wantErr: true,
+		},
+		{
+			name: "Single group permission",
+			args: args{
+				r: &APIResponse{
+					Values: map[string]interface{}{
+						"values": []interface{}{map[string]interface{}{
+							"group": map[string]interface{}{
+								"name": "group1",
+							},
+							"permission": "REPO_ADMIN",
+						}},
+					},
+				},
+			},
+			want: []GroupPermission{
+				GroupPermission{
+					Group: Group{
+						Name: "group1",
+					},
+					Permission: PermissionRepositoryAdmin.String(),
+				},
+			},
+			wantErr: false,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got, err := GetGroupsPermissionResponse(tt.args.r)
+			if err != nil && !tt.wantErr {
+				t.Errorf("GetGroupsPermissionResponse() error = %v, wantErr %v", err, tt.wantErr)
+				return
+			}
+			if !reflect.DeepEqual(got, tt.want) {
+				t.Errorf("GetGroupsPermissionResponse() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}
+
 func TestNewAPIResponse(t *testing.T) {
 	type args struct {
 		r *http.Response