Add SetUser extension to IQueryRequestBuilder

[tobias-tengler]

We already created such an extension in our own project, but I was reminded again today by Michael's video that it would probably be a good idea to expose this method as part of the library.
I called the method SetUser to have a symmetry to the already existing GetUser method on the IResolverContext.

Example:

var request = QueryRequestBuilder.New()
    .SetQuery("{ me { name } }")
    .SetUser(new ClaimsPrincipal())
    .Create();