Use realtime agent beyond user_agent in swarm

[sonichi]

          The realtime agent is used as a user proxy here. Fundamentally, can realtime agent be used also in one of the swarm agents that's not user_agent? Shall we allow users to decide where it's used?

cc @marklysze @yiranwu0 @yanenli2 @davorrunje @sternakt

Originally posted by @sonichi in #196 (comment)

The reason I said it doesn't have to be the user_agent in swarm is that we allow developers to define their own user proxy agents without using the user_agent argument. In that case, realtime agent can be a drop-in replacement of the user proxy agent I think.