Update Agent server RPC functions to include agent ID for agent runtime with multiple agents

## Motivation

- Enable [Backend.AI](http://Backend.AI) agent server to support multiple agents within a single physical agent process, now called ‘Agent Runtime’, allowing different scheduling units and backend configurations while maintaining backward compatibility with existing single-agent deployments.

## Required Features

- Agent: create multiple agents based on the given config
- Agent: modify RPC functions to take in agent ID from the client
- Manager: Add agent ID on call sites of the agent RPC functions

## Impact

- Should work the same for existing single-agent configurations

## Testing Scenarios

- Add unit tests
- Run tests on local instance
- If possible, run multiple agent servers and test if agent runtime routing works properly

JIRA Issue: BA-2753