text: Title: Provide user-facing functionality for fixing serialization/deserialization of custom scalars
  Body: When parsing the textual representation of a GraphQL schema into a schema object, custom scalars' serialization and deserialization code are not picked up since they are not part of the textual representation. This causes issues when working with custom scalar objects.
  
  #388 fixes this problem for our test suite. We need to expose a similar mechanism that users of the package can use to resolve this problem for their use cases.