Provide a way to dynamically configure the set of sub-agents for the model

Similarly to tools, model's sub_agents are often need to be dynamically configured: based on the session's state, current user's permissions or some other factors. Tools can be dynamically configured using Toolsets but dynamically configuring sub_agents require creating custom plugins.

Providing SubagentSet, or a similar class, with the API similar to Toolset should simplify agent development.