Support dynamic Model Provider

[growmuye]

I encountered a problem: a single LLM service provider may occasionally experience fluctuations, causing the online Agent to crash. Currently, the idea is to provide backup model service providers to ensure online stability.

I noticed that the model parameter of the SDK cannot be dynamically constructed like the instruction. I hope the ADK official can consider this matter.

Other thoughts: I'm not sure yet whether parameters like top_p and temperature need to support dynamics, but if we want to ensure the effectiveness of the Prompt, the same Prompt may require different parameter configurations on different models.

[trofoto]

Good idea. Besides this scenario, when using adk-web, I hope to be able to delay configuring the model because it is obtained from a service, and the API is an async. Currently, I haven't found a suitable way to obtain model information in async mode when creating an agent.