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"
/>

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

packages/twenty-server/src/engine/workspace-cache/types/workspace-cache-key.type.ts

@@ -17,6 +17,7 @@ export const WORKSPACE_CACHE_KEYS_V2 = {
   flatIndexMaps: 'flat-maps:index',
   flatViewMaps: 'flat-maps:view',
   flatViewFieldMaps: 'flat-maps:view-field',
+  flatViewFieldGroupMaps: 'flat-maps:view-field-group',
   flatViewGroupMaps: 'flat-maps:view-group',
   flatViewFilterMaps: 'flat-maps:view-filter',
   flatViewFilterGroupMaps: 'flat-maps:view-filter-group',