Move `ISession` and perhaps `WebSocketAcceptContext` up into the root namespace

[dougbu]

This interface and class are currently in the Microsoft.AspNetCore.Http.Features but are exposed much more broadly e.g. HttpContext contains a property of type ISession. They should be in the Microsoft.AspNetCore.Http namespace.

[dougbu]

• 6f24508
• aspnet/MusicStore@0cdc7f7
• aspnet/Session@5371e13
• aspnet/WebSockets@0f9e4eb