support multipart/form-data in router

[irisdingbj]

router is not able to handle multipart/form-data:

kubectl exec -it client-test-87d6c7d7b-wvxf5 -n chatqa -- curl http://router-service-data-prep.chatqa.svc.cluster.local:8080 -X POST -H 'Content-Type: multipart/form-data' -F 'link_list=["https://istio.io", "https://opea.dev"]'
{"detail":"Must provide either a file or a string list."}

[irisdingbj]

close this as fix has been merged