[HEP] Improve the e2e testing for plugins like AI Proxy/AI Cache that depend on LLM

[johnlanni]

Why do you need it?

Because running model services in an e2e environment or accessing the LLM provider's API is not very feasible. The e2e testing of plugins for the related logic was not thorough before, which would lead to defects in subsequent iterations.

How to implement it?

• Implement an LLM mock server that supports mocking all LLM vendor protocols #1630
• Enhance the e2e testing of the ai-proxy plugin based on the LLM mock server #1629
• Enhance the e2e testing of the ai-cache plugin based on the LLM mock server #1631
• Enhance the e2e testing of the ai-statistics plugin based on the LLM mock server #1632
• Enhance the e2e testing of the ai-token-ratelimit plugin based on the LLM mock server #1633
• Enhance the e2e testing of the ai-quota plugin based on the LLM mock server #1634
• Implement a mock service for Alibaba Cloud Content Security #1635
• Enhance the e2e testing of the ai-security-guard plugin based on the LLM mock server and Alibaba Cloud Content Security #1636