Added loader pluggins to handle openapi.
Added loader pluggins to handle openapi.

jhendrixMSFT/autorest,jhendrixMSFT/autorest,Azure/autorest,jhendrixMSFT/autorest,fearthecowboy/autorest,Azure/autorest,Azure/autorest,fearthecowboy/autorest,Azure/autorest,jhendrixMSFT/autorest