Fix apollo-server-express context TypeScript definition

[KATT]

Noticed an issue when updating apollo-server-express today - my context function started breaking.

It looked something like this

interface MyContext {
  token?: string;
}
new ApolloServer({
  typeDefs,
  resolvers,
  context: (): MyContext => {
    return {
      // ...
    };
  },
});

It seem to come from #2330 which enforces the output of the ContextFunction to be the same as the first arg to the function, which I think is wrong.

This PR allows you the ContextFunction to have an incoming and an outgoing context and let's you return whatever value you want in your apollo-server-express context.

---

TODO:

• Update CHANGELOG.md with your change (include reference to issue & this PR)
• 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

[apollo-cla]

@KATT: Thank you for submitting a pull request! Before we can merge it, you'll need to sign the Meteor Contributor Agreement here: https://contribute.meteor.com/

[KATT]

Alright, this was also in #2350