[Feature]: Support building agents from an AgentConfig (YAML) file

Hi all, could AgentScope support an ADK-style AgentConfig approach for constructing agents?

In ADK, agents can be created not only through code, but also directly from YAML configuration files. References:

https://google.github.io/adk-docs/agents/config/
https://github.com/google/adk-python/blob/10bdc078a453102a69afcb02e5a5665353eda11f/contributing/samples/multi_agent_basic_config/root_agent.yaml
This is a very convenient workflow: almost anyone can quickly build a multi-agent system this way. As model capabilities improve, we believe config-driven agents can perform comparably to agents built purely with code.

I’d like to propose adding support for launching/building an agent from an AgentConfig YAML file.
Example:
```yaml
# Optional: SequentialAgent, LoopAgent
agent_class: LlmAgent
model: qwen3-coder-plus
name: root_agent
description: Learning assistant that provides tutoring in code and math.
instruction: |
You are a learning assistant that helps students with coding and math questions.
sub_agents:
- config_path: search_agent.yaml
tools:
builtInTools:
- search
- codeExecutor
mcp:
- url: https://alipay
apiKey: xxx
- https://amap
```
Looking forward to your thoughts—thanks!