RFC: Define custom scalars in terms of built-in scalars.

[IvanGoncharov]

As @leebyron suggested during last WG I'm adopting #914 and rebasing it on top of current master.

Here is the original description:

This proposes an additive change which allows custom scalars to be defined in terms of the built-in scalars. The motivation is for client-side code generators to understand how to map between the GraphQL type system and a native type system. As an example, a URL custom type may be defined in terms of the built-in scalar String. It could define additional serialization and parsing logic, however client tools can know to treat URL values as String. Presently, we do this by defining these mappings manually on the client, which is difficult to scale, or by giving up and making no assumptions of how the custom types serialize.

Another real use case of giving client tools this information is GraphiQL: this change will allow GraphiQL to show more useful errors when a literal of an incorrect kind is provided to a custom scalar. Currently GraphiQL simply accepts all values.

To accomplish this, this proposes adding the following:

• A new property when defining GraphQLScalarType (ofType) which asserts that only built-in scalar types are provided.

• A second type coercion to guarantee to a client that the serialized values match the ofType.

• Delegating the parseLiteral and parseValue functions to those in ofType (this enables downstream validation / GraphiQL features)

• Exposing ofType in the introspection system, and consuming that introspection in buildClientSchema.

• Adding optional syntax to the SDL, and consuming that in buildASTSchema and extendSchema as well as in schemaPrinter.

• Adding a case to findBreakingChanges which looks for a scalar's ofType changing.