[apollo-server-plugin-base] fix GraphQLRequestListener type definitions

[doomsower]

This makes typescript compiler happy when listers return void:

export class SomeListener implements GraphQLRequestListener {
  parsingDidStart() {
    console.log('parsingDidStart');
  }
}

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

[abernix]

Thanks!