Rust: Allow Agents to hold onto a Client and vice versa

- Replace ClientSideConnection/AgentSideConnection structs with type aliases to RpcConnection
- RpcConnection::new now accepts a handler factory FnOnce(Rc<Self>) and returns Rc<Self>

Before, there wasn't a way for an Agent to hold onto a way to
communicate with the Client and vice versa.

Now they can. This matches the behavior in the TS SDK

Signed-off-by: Ben Brandt <benjamin.j.brandt@gmail.com>

Author: benbrandt