[Voltagent] Add support for distributed subagents with routing

### Is your feature request related to a problem? Please describe. This is the ability to add external voltagent instances as subagents I'm exploring the possibility of running multiple Voltagent instances across different services/projects, acting as subagents that can route requests between each other using HTTP or Pub/Sub. The idea is to create a flexible, modular agent architecture. Here's an example of how an external agent might be registered: ` registerExternalAgent({ name: "agent2", description: "Use this agent for weather-related queries", instructions: "", http: "http://localhost:3412/" }); ` I’ve implemented a basic prototype that uses a classification prompt to determine which agent/tool to invoke. It works, but: It’s verbose also passing context (like chat history or user state) is clunky and not scalable. ` export const classifierAgent = new Agent({ memory: memory, id: "classifier-agent", name: "Classifier Agent", description: "Classifies the user's