added initial configuration for GraphQL

[jtorreguitar]

Summary:

GraphQL is a new paradigm to replace REST APIs which makes for a more flexible way to make requests. To learn more about them go to this link.

This PR hopes to take the initial steps to be able to, in future projects, use a GraphQL API. Right now it only "replicates" the functionality to get users by id or to get all users. Of course with the caveat that one can get subsets of the user data. For example, going to https://localhost:5001/ui/playground (if you are running your project in the local port 5001) you can write the following query:

{
  users
  {
    userName
  }
}

This will return a list of all usernames in the application. Right now getting emails will fail because the GraphQL API seems to default to them being non-nullable. Haven't found out how to change this yet.

The tutorial that was used to set this up was: https://medium.com/swlh/the-no-bullsh-t-guide-to-graphql-in-net-core-9df290be7f27
Some corrections had to be made from this tutorial as well: https://medium.com/volosoft/building-graphql-apis-with-asp-net-core-419b32a5305b