[Draft] Add an AgentToolset

[strawgate]

Relating to #2329

Some stuff to look at re: Agent Handoff or AgentToolset

1. Add Handoff method to Agent (can take an Agent with the same deps, deps + input model, or different deps)
2. Add an AgentToolset that allows more customization for Agent handoffs

It's not fully wired up but ex:

inner_agent: Agent[InnerAgentDeps, str] = Agent[InnerAgentDeps, str]('inner', deps_type=InnerAgentDeps)

other_outer_agent: Agent[OuterAgentDeps, str] = Agent[OuterAgentDeps, str]('other_outer', deps_type=OuterAgentDeps)

outer_agent: Agent[OuterAgentDeps, str] = Agent[OuterAgentDeps, str]('outer', deps_type=OuterAgentDeps)


async def outer_to_inner_deps_func(deps: OuterAgentDeps, input: str) -> InnerAgentDeps:
    """Transform outer deps to inner deps."""
    return InnerAgentDeps(client=deps.client, extra=input)


outer_agent.handoff(agent=inner_agent, deps_func=outer_to_inner_deps_func)
outer_agent.handoff(agent=other_outer_agent)