Built-in support for HTTP header to GraphQLContext copying

[rstoyanchev]

A WebGraphQlInterceptor can copy HTTP request into the GraphQLContext, and those values are then accessible in controller methods via @ContextValue. There is an example of this in the docs, but we can make it easier by providing a built-in interceptor configurable with HTTP header names to transfer and optional target keys to map to in the GraphQL context.