Add agent server management functions

[pan-x-c]

Description

In the old version, the rpc agent server only has a call_func method, and all agents' own methods and agent server management related methods need to be implemented through the call_func, making this method bloated.

This PR adds a series of agent server management functions (used to support agent server management modules), and extracts management related methods from the original call_func interface.

All agent server management functions are listed below:

• is_alive: whether the server is alive
• stop: stop the server
• create_agent: create a new agent on the server
• delete_agent: delete agents from the server
• clone_agent: clone an agent with specific agent_id
• get_agent_id_list: get agent_id of all agents on the server as a list
• get_agent_info: get the agent information of the specific agent_id
• get_server_info: get the resource utilization information of the server
• set_model_configs: update the model configs of the server
• get_agent_memory: get the memory of a specific agent on the server
• download_file: download a file from the server

Each method above is implemented independently and doesn't use the original call_func method.

The original call_func is renamed to call_agent_func, and only handle method calls of the agent itself.
The placeholder update method is also extracted from the original call_func method as a separate update_placeholder method.

This PR makes the rpc messages used by each method more compact. Tests show that it can reduce the running time of large-scale simulation example by 5 - 10%.

Unittests and tutorials have been updated accordingly.

Checklist

Please check the following items before code is ready to be reviewed.

• Code has passed all tests
• Docstrings have been added/updated in Google Style
• Documentation has been updated
• Code is ready for review