#6094 Prevent creating a custom field with an existing name (#6100)

Fixes #6094 
Description: Added logic inside SettingsObjectNewFieldStep2.tsx to
prevent form submission
Current Behaviours:
<img width="947" alt="Screenshot 2024-07-03 at 1 45 31 PM"
src="https://github.com/twentyhq/twenty/assets/95612797/bef54bc4-fc83-48f3-894a-34445ec64723">

---------

Co-authored-by: Marie Stoppa <marie.stoppa@essec.edu>

packages/twenty-front/src/modules/object-metadata/validation-schemas/objectMetadataItemSchema.ts

@@ -10,7 +10,7 @@ export const objectMetadataItemSchema = z.object({
   createdAt: z.string().datetime(),
   dataSourceId: z.string().uuid(),
   description: z.string().trim().nullable().optional(),
-  fields: z.array(fieldMetadataItemSchema),
+  fields: z.array(fieldMetadataItemSchema()),
   icon: z.string().startsWith('Icon').trim(),
   id: z.string().uuid(),
   imageIdentifierFieldMetadataId: z.string().uuid().nullable(),
@@ -19,8 +19,8 @@ export const objectMetadataItemSchema = z.object({
   isRemote: z.boolean(),
   isSystem: z.boolean(),
   labelIdentifierFieldMetadataId: z.string().uuid().nullable(),
-  labelPlural: metadataLabelSchema,
-  labelSingular: metadataLabelSchema,
+  labelPlural: metadataLabelSchema(),
+  labelSingular: metadataLabelSchema(),
   namePlural: camelCaseStringSchema,
   nameSingular: camelCaseStringSchema,
   updatedAt: z.string().datetime(),