Introducing view field group syncable entity (#17867)

## Context
Introduces a new viewFieldGroup entity that allows grouping view fields
into sections (e.g. "General", "Additional", "Other") within a view.

The page layout fields widget needs a way to organize fields into
sections. Today, views have no concept of field grouping. This PR
introduces the viewFieldGroup entity which sits between a view and its
viewFields, enabling section-based organization.

<img width="401" height="724" alt="Layout - V2 (customize visibility)"
src="https://github.com/user-attachments/assets/6376e2ab-44db-42bf-9d2c-758f56f6b548"
/>

---------

Co-authored-by: Charles Bochet <charles@twenty.com>

packages/twenty-server/test/integration/utils/view-test.util.ts

@@ -1,3 +1,4 @@
+import { type ViewFieldGroupDTO } from 'src/engine/metadata-modules/view-field-group/dtos/view-field-group.dto';
 import { type ViewFieldEntity } from 'src/engine/metadata-modules/view-field/entities/view-field.entity';
 import { type ViewFilterGroupEntity } from 'src/engine/metadata-modules/view-filter-group/entities/view-filter-group.entity';
 import { ViewFilterGroupLogicalOperator } from 'src/engine/metadata-modules/view-filter-group/enums/view-filter-group-logical-operator';
@@ -23,6 +24,22 @@ export const assertViewStructure = (
   }
 };
 
+export const assertViewFieldGroupStructure = (
+  viewFieldGroup: ViewFieldGroupDTO,
+  expectedFields?: Partial<ViewFieldGroupDTO>,
+) => {
+  expect(viewFieldGroup).toBeDefined();
+  expect(viewFieldGroup.id).toBeDefined();
+  expect(viewFieldGroup.name).toBeDefined();
+  expect(viewFieldGroup.viewId).toBeDefined();
+  expect(typeof viewFieldGroup.position).toBe('number');
+  expect(typeof viewFieldGroup.isVisible).toBe('boolean');
+
+  if (expectedFields) {
+    expect(viewFieldGroup).toMatchObject(expectedFields);
+  }
+};
+
 export const assertViewFieldStructure = (
   viewField: ViewFieldEntity,
   expectedFields?: Partial<ViewFieldEntity>,