Context Propagation to DataFetcher's with Spring MVC

[rstoyanchev]

For Spring Security support on Spring MVC, we'll need context propagation.

Initially the request is on a Servlet container thread with ThreadLocal context, then it passes through the Reactor web interception chain before getting to GraphQL where DataFetchers are invoked and those would be expected to be imperative but also potentially reactive.

We'll need to assist with context propagation for this to work seamlessly. It will probably involve some mechanism that enables Spring Security or others to help decide which ThreadLocal context to propagate.