Support merging of schema directives

[freiksenet]

Add support to merging directive definitions in mergeSchemas.

TODO:

• If this PR is a new feature, reference an issue where a consensus about the design was reached (not necessary for small changes)
• Make sure all of the significant new logic is covered by tests
• Rebase your changes on master so that they can be merged easily
• Make sure all tests and linter rules pass
• Update CHANGELOG.md with your change. Include a description of your change, link to PR (always) and issue (if applicable). Add your CHANGELOG entry under vNEXT. Do not create a new version number for your change yourself.

[Torsten85]

I'm looking forward to this feature and tried your PR, but I get the following error:

const schema = makeExecutableSchema({
  typeDefs: `

    directive @auth(role: Role = ADMIN) on OBJECT | FIELD_DEFINITION

    enum Role {
      ADMIN
      USER
      PUBLIC
    }

    type User {
      firstname: String @auth(role: PUBLIC)
      lastname: String
    }

    type Query {
      user: User
    }
  `
});

const merged = mergeSchemas({ schemas: [schema], mergeDirectives: true });

Schema must contain unique named types but contains multiple types named "Role".

[freiksenet]

@Torsten85 thanks a lot, I'll check it and add fix+test for it.

[freiksenet]

@Torsten85 Fixed, is it working for you?

[mfix22]

LGTM 👍@hwillson should confirm too though

[somil-nagarro]

I too waiting for this PR to get merge, please let me know ETA

[andrew-makarenko]

@benjamn @hwillson could you please take a look?
It's a blocker for me as well. Thank you!

[hwillson]

Thanks very much for working on this @freiksenet!