Implement "roots" capability support + extract some shared logic

[jakemac53]

See https://spec.modelcontextprotocol.io/specification/2025-03-26/client/roots/

Refactor details

• Added the MCPBase class now that both clients and servers can handle requests, and deal with progress notifications for those, etc.
  • Hides the _peer from the other classes, exposing instead its own registerMethodHandler and registerNotificationHandler, sendRequest and sendNotification.
• The MCPClient class also now has a similar mixin pattern to the MCPServer class, with RootsSupport being the first one.
• Added an initialize lifecycle method for clients, which is called in the base class constructor. This allows mixins to run code on construction of the object - specifically this is useful for updating the capabilities object for the client, which must be done prior to connecting to any servers.