GraphQLEngine - make _documentExecutor passable through the constructor

[K-Pavlov]

Currently the IDocumentExecuter in GrapQLEngine is a readonly field which is set to a concrete type:
private readonly IDocumentExecuter _documentExecutor = new GraphQL.DocumentExecuter();
This is not ideal as DocumentExecuter gives us the ability to control the IExecutionStrategy:
protected virtual IExecutionStrategy SelectExecutionStrategy(ExecutionContext context)
which is currently not possible when using the GrapQLEngine

[tlil]

👍 Merged. Thanks!