"isValueOfTheType" field should be mandatory for "custom" scalar graphql types

[olegrok]

Seems we should add a check here https://github.com/tarantool/cartridge/blob/master/cartridge/graphql/types.lua#L101 as it was done for other options. As it's easy to miss its declaration and get such error - https://github.com/tarantool/cartridge/blob/master/cartridge/graphql/validate_variables.lua#L99