Allow schemaPrinter to be customized / extended.

Some libraries are having to copy the schemaPrinter code to do custom stuff to the output, (@apollo/federation, type-graphql). These libraries require outputting type and field level directives since this isn’t supported in this library. Is there any support to do so, or can the schemPrinter just export the other functions so we don’t have to copy every function or, even better, create a SchemaPrinter class so everything can be over-ridden, then printSchema options can set the printer for BC:

type Options = {
    commentDescriptions?: boolean,
    printer?: SchemaPrinter
};

const defaultPrinter = new SchemaPrinter();

export function printSchema(schema: GraphQLSchema, options?: Options): string {
    const printer = options.printer || defaultPrinter;

    return printer.print(
        schema,
        n => !isSpecifiedDirective(n),
        isDefinedType,
        options,
    );
}

export function printIntrospectionSchema(
    schema: GraphQLSchema,
    options?: Options,
): string {
    const printer = options.printer || defaultPrinter;

    return printer.print(
        schema,
        isSpecifiedDirective,
        isIntrospectionType,
        options,
    );
}

[spawnia]

How about we add a config option:

printDirectives: (definition: GraphQLType|GraphQLField|GraphQLEnumValue|GraphQLInputField|GraphQLArgument) => string

Whenever a definition that can hold directives is encountered, the given function would be called with it and is free to print any directives.

[mpiroc]

@IvanGoncharov What if printSchema only printed directives that were not visited during schema construction? Using your example:

type Foo {
  foo: String
}

type Bar @copyFields(fromType: "Foo") {
  bar: String
}

// 1
printSchema(makeExecutableSchema({
    typeDefs,
    schemaDirectives: {
        copyFields: CopyFieldsVisitor
    }
}))

// 2
printSchema(makeExecutableSchema({
    typeDefs,
    schemaDirectives: {
    }
}))

The first snippet would print:

type Foo {
  foo: String
}

type Bar {
  foo: String
  bar: String
}

The second would print:

type Foo {
  foo: String
}

type Bar @copyFields(fromType: "Foo") {
  bar: String
}