[GraphQL Plugin] `viewer` query

[Taras Mankovski]

Motivation

As a developer writing features that use the GraphQL API provided by the GraphQL Plugin, I need to be able to query for data of a user who's using the application. For example, I should be able to retrieve my components using the following query.

{
  viewer {
    components {
       id
    }
  }
}

This query should return components that are owned by the user who's querying.