Bugfix/add field

[eric-burel]

@EloyID it should make "addField" more consistent with "extendCollection" and avoid the weird issue when we use both. Under the hood, "addField" is just "extendCollection(collection, {schema})" so from now on you might want to use extendCollection only (try to call it only once per package to avoid extra computation on app startup + split between client and server when you have resolveAs, si computedPermissions for example in the platform)