feat: ai-rag plugin

[shreemaan-abhishek]

Description

Implement the ai-rag plugin that uses RAG (Retrieval Augmented Generation), to return information about untrained events/facts from LLMs.

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)