chore: Expose contact identifier in API (#2391)

app/views/api/v1/models/_contact.json.jbuilder

@@ -4,6 +4,7 @@ json.email resource.email
 json.id resource.id
 json.name resource.name
 json.phone_number resource.phone_number
+json.identifier resource.identifier
 json.thumbnail resource.avatar_url
 json.custom_attributes resource.custom_attributes
 json.conversations_count resource.conversations_count if resource[:conversations_count].present?