message: Add model definitions for PermissionOffer and PermissionOfferResponse
  This accomplishes a few things:
  Prevents ROS from complaining
  Allows us to leverage the object store level permissions code in the future to implement permission offer/response at this layer
diff: @@ -251,12 +251,34 @@ SharedRealm Permissions::management_realm(std::shared_ptr<SyncUser> user, const
  Property{"userId", PropertyType::String},
  Property{"metadataKey", PropertyType::String|PropertyType::Nullable},
  Property{"metadataValue", PropertyType::String|PropertyType::Nullable},
  - Property{"metadataNamespace", PropertyType::String|PropertyType::Nullable},
  + Property{"metadataNameSpace", PropertyType::String|PropertyType::Nullable},
  Property{"realmUrl", PropertyType::String},
  Property{"mayRead", PropertyType::Bool|PropertyType::Nullable},
  Property{"mayWrite", PropertyType::Bool|PropertyType::Nullable},
  Property{"mayManage", PropertyType::Bool|PropertyType::Nullable},
  - }}
  + }},
  + {"PermissionOffer", {
  + Property{"id", PropertyType::String, Property::IsPrimary{true}},
  + Property{"createdAt", PropertyType::Date},
  + Property{"updatedAt", PropertyType::Date},
  + Property{"expiresAt", PropertyType::Date},
  + Property{"statusCode", PropertyType::Int|PropertyType::Nullable},
  + Property{"statusMessage", PropertyType::String|PropertyType::Nullable},
  + Property{"token", PropertyType::String|PropertyType::Nullable},
  + Property{"realmUrl", PropertyType::String},
  + Property{"mayRead", PropertyType::Bool},
  + Property{"mayWrite", PropertyType::Bool},
  + Property{"mayManage", PropertyType::Bool},
  + }},
  + {"PermissionOfferResponse", {
  + Property{"id", PropertyType::String, Property::IsPrimary{true}},
  + Property{"createdAt", PropertyType::Date},
  + Property{"updatedAt", PropertyType::Date},
  + Property{"statusCode", PropertyType::Int|PropertyType::Nullable},
  + Property{"statusMessage", PropertyType::String|PropertyType::Nullable},
  + Property{"token", PropertyType::String|PropertyType::Nullable},
  + Property{"realmUrl", PropertyType::String},
  + }},
  };
  config.schema_version = 0;
  auto shared_realm = Realm::get_shared_realm(std::move(config));
  