Support for Agent Tools, to a wrap the invocation of an agent via a tool

[ekzhu]

What feature would you like to be added?

We can create an agent wrapper AgentTool that takes an agent ID, the RPC definition (i.e., request message type and response message type), and create a Tool object, similar to how FunctionTool is used. This allows agent to be wrapped as tool. Needs a bit more thinking about how to put the agent's reference to the runtime into the Tool object.

Why is this needed?

In many scenarios, agent themselves are tools with a natural language interface.