[Flight] Make it possible to use native gRPC Java APIs, and/or simplify the implementation

[lidavidm]

Describe the enhancement requested

grpc/grpc-java#10318 introduces a new type of blocking stub in grpc-java which handles streaming calls. This could drastically simplify DoGet/DoPut/DoExchange which try to present blocking interfaces to nonblocking APIs. However, preferably we would make it possible to use gRPC APIs directly instead of making users wait for us to wrap new APIs in even less documented new APIs. Also, it's not clear that this simplifies the server side at all (unfortunately).