Support for c# extensions

[john-obrien]

This PR introduces support for C# extensions to add to the definition of a query type.

This is particularly useful when the schema is defined across several projects / assemblies and you wish to stitch these together into a single graph.

// Actors.csproj
class Actor
{
   public Guid Id { get; set; }
   public string Name { get; set; }
}

// Movies.csproj
class Movie
{
    public Guid Id { get; set; }
    public string Title { get; set; }
}

// Schema.csproj
static class MovieExtensions
{
    public async Task<Actor[]> Actors(this Movie movie, [Inject] IActorRepository repository)
       => await repository.GetActorsPerMovie(movie.Id);
}
class Query
{
    public async Task<Movie> Movie([Inject] IMovieRepository repository, Guid id)
       => await repository.GetMovie(.id);
}

var response = await RequestHandler
  .New()
  .WithQuery<Query>()
  .WithQueryExtensions(typeof(MovieExtensions))
  .Generate();

[tlil]

Can bring the null-check down to the extension method:

public static bool IsExtensionMethod(this MethodInfo methodInfo) => 
    methodInfo?.IsDefined(typeof(ExtensionAttribute), false) ?? false;

And thus, this line could be removed and the property getter expressed like so:

public bool IsExtensionMethod => (AttributeProvider as MethodInfo).IsExtensionMethod()

#nitpick