GraphQL.js emulation: expand to resolveType and isTypeOf

[benjie]

resolveType and isTypeOf in Grafast currently (or will once #2301 is closed) only support (value: any) => string | GraphQLObjectType call signature. GraphQL.js supports (value: any, context: any, resolveInfo: GraphQLResolveInfo, abstractType: GraphQLUnionType | GraphQLInterfaceType) => PromiseOrValue<string | GraphQLObjectType> signature. Ideally we don't want Grafast having to handle all the complexities of this for most Grafast schemas, however if the schema includes GraphQL.js emulation then we will need to. We should therefore add a flag such as emulateGraphQLJS?: boolean that enables resolveType and isTypeOf to run in GraphQL.js emulation mode (with the additional properties) - we'd accomplish this by running resolveType as a step much like the graphqlResolver, that way it can be satisfied asynchronously if needed.

We should also add a flag to the relevant extensions: unionType.extensions.grafast.emulateGraphQLJS; if set true then we'd emulate GraphQLJS for the type, if false we wouldn't, and if unset we'd default to the global emulateGraphQLJS flag passed to the grafast() function (or via the preset or whatever).