Restructure RPC

[jackgerrits]

RPC can be built as a client side concept entirely to greatly simplify the runtime.

• send_message can be removed from AgentRuntime
• BaseAgent.send_message should be renamed to send_rpc - its semantics remain identical
• Remove RPC code from runtime impl, protos, etc

Procedure for an RPC is as follows:

1. Request ID (a uuid) is generated as request_id
2. Topic ID to be sent to is generated as recipient_agent_id.type and potentially some extra distinguishing info request_topic_id
3. Topic ID to be received from is generated as a well known format that contains the recipient_agent_id.type and request_id as response_topic_id
4. An empty Future is generated
5. Explicit subscription is generated for topic_id which does two things: sets the future to the received message, removes the subscription
6. Event is published to request_topic_id with request_id
7. Future is returned from the function for the consumer to await

Note:

• I think we should potentially make the per agent implicit rpc subscription a bit more specific to avoid surprising behavior of agents receiving publishes without manually adding a subscription