This enables generating generic service stubs for all the data-plane-api
proto services when generating Java classes with protoc.
This is generally not needed when implementing a gRPC server but in our case we're implementing
it behind our legacy protobuf RPC framework which rely on these stubs. As far as I know the only negative
with enabling these is generating some potentially unnecessary Java classes.
Signed-off-by: Snow Pettersen <snowp@squareup.com>