Appian Custom Connected System to consume gRPC based service

Hi, 

Im engaged with a client who has adopted in their enterprise standards that all exposed microservices within their organization be consumed using gRPC (https://grpc.io/).   Some  of those services will need to be consumed by the Appian solutions we are planning.    My current understanding of gRPC & supported authentication mechanism is the out of the box  Appian connected system, HTTP & Open API authentication types will not work.   As such we are considering if building custom connected systems and support for integration objects might work. 

Has anyone in the community run across a requirement like this and done any successful custom connected system development to support the gRPC framework or solved the integration needs  in some other fashion?

Also with Appian Integration SDK when developing custom Connected systems is it a requirement to use one of the Authentication Types supported by Appian (https://docs.appian.com/suite/help/21.4/connected_system_authentication.html)  or can a custom connected system implement a custom authentication model as well?

Thanks for any suggestions you can provide.

Bob