Chatting with custom endpoint throws cors error if cors is disabled in endpoint server

[vishnoianil]

If the endpoint server doesn't enable cors, qa.ui.instructlab.ai and ui.instructlab.ai both will hit the folllowing error

Access to fetch at 'https://XXXX/v1/chat/completions' from origin 'https://qa.ui.instructlab.ai/' has been blocked by CORS policy: Response to preflight request doesn't pass access control check: No 'Access-Control-Allow-Origin' header is present on the requested resource. If an opaque response serves your needs, set the request's mode to 'no-cors' to fetch the resource with CORS disabled.

cc: @mingxzhao