Fix flat entity maps date serialization (#16420)

Changes:
- as we store date in redis as serialized, let's make all flatEntity
dates as string. This requires changing FlatEntity types and making sure
that entity are converted to flatEntity and flatEntity to dtos

packages/twenty-server/src/engine/metadata-modules/flat-object-metadata/__mocks__/get-flat-object-metadata.mock.ts

@@ -9,7 +9,7 @@ type FlatObjectMetadataOverrides = Required<
 export const getFlatObjectMetadataMock = (
   overrides: FlatObjectMetadataOverrides,
 ): FlatObjectMetadata => {
-  const createdAt = '2024-01-01T00:00:00.000Z' as unknown as Date;
+  const createdAt = '2024-01-01T00:00:00.000Z';
 
   return {
     viewIds: [],