We need to add Excel file reading to the RAG module. Currently, agentscope.rag provides readers for text…

We need to add Excel file reading to the RAG module. Currently, agentscope.rag provides readers for text…: a task in LegoFlow-SWE (Harbor dataset). The new reader should be accessible via from agentscope.rag import ExcelReader and behave consistently with the existing reader family.