Allows to add schema definition missing in the original schema

[IvanGoncharov]

Split out from #1438

Allows to inject directives and types into SDL-based schema:

import { GrapQLDateTime } from 'somepackage';
let schema = new GraphQLSchema({
  types: [GrapQLDateTime],
});

const ast = parse(`
  type Query {
    timestamp: DateTime
  }
`);
schema = extendSchema(schema, ast);

let schema = new GraphQLSchema({
  directives: [
    new GraphQLDirective({
      name: 'onSchema',
      locations: ['SCHEMA'],
    }),
  ],
});

const ast = parse(`
  schema @onSchema {
    query: Foo
  }
  type Foo {
    bar: String
  }
`);
schema = extendSchema(schema, ast);

[IvanGoncharov]

@mjmahone Based on the discussion in #1441 I'm not sure if you approved that change or not. Can you please review it separately?

[mjmahone]

This looks good.

[mjmahone]

oh wait just saw this: can we do instead:

if (schemaDef) {
  schemaExtensions.push(schemaDef);
}
for (const schemaExtension of schemaExtensions) {

[IvanGoncharov]

@mjmahone You are right, the current code is confusing 😞
Sadly, schemaExtensions used bellow explicitly for extension nodes.
Moreover schemaDef doesn't need schemaDef.operationTypes because it always initialized.
So I decided to just duplicate code since #1435 will reduce this code and we can simplify things in subsequent refactorings.

[IvanGoncharov]

@mjmahone Can you please review updated PR?

[IvanGoncharov]

I initially forgot to add this check, so I also change tests to cover it.

[IvanGoncharov]

tiny refactoring 😜

[IvanGoncharov]

After #1435 I will move this check into SDL validation.

[mjmahone]

This seems really good now!