Enable easy customization of GraphQL execution

[kaqqao]

In order to customize any of the following:

• Building ExecutionInput
• Executing the operation
• Handling the response

it is currently necessary to override the whole controller and/or WebSocket handler.

To facilitate easier customization, all of these concerns are now delegated to GraphQLExecutor implementation that can easily be swapped by the user.

Right now, 3 built-in implementations exist (with at least 1 more to come):

• servlet.DefaultGraphQLExecutor - used by the GraphQL controller in a servlet-based app
• reactive.DefaultGraphQLExecutor - used by the GraphQL controller in a reactive app
• servlet.websocket.DefaultGraphQLExecutor - used by the WebSocket handler in a servlet-based app

Once WebSockets are supported on the reactive stack, an additional executor will be added.