Make EverythingServer capable of running either stdio or http

[mikekistler]

From this comment on #709 by @halter73:

It might be interesting to demonstrate how you can share code for an HTTP and stdio everything server. We could create EverythingServer.Http, EverythingSerever.Stdio and EverythingServer.Core projects. And the Core project could have an AddEverythingMcpHandlers(this IMcpServerBuilder builder, ConcurrentDictionary<string, ConcurrentDictionary<string, byte>> subscriptions) method. It might even make sense to add something like public class SubscriptionCollection : ConcurrentDictionary<string, ConcurrentDictionary<string, byte>> to the core project.

The only thing that couldn't be shared is Program.cs. The Stdio project would use hosted services to manage subscriptions, and the Http project would do the RunSessionHandler logic.