Generating grpc descriptors requires `protokt-runtime-grpc-lite-jvm` on the classpath

When generating grpc descriptors with ``` protokt { generate { grpcDescriptors = true } } ``` the generated descriptor code will contain references to `protokt.v1.grpc.KtMarshaller` and `protokt.v1.grpc.SchemaDescriptor`, from `protokt-runtime-grpc-lite-jvm`, so that jar need to be on the classpath to be able to compile. Maybe this should be added to the documentation, or perhaps the gradle plugin could add it to the classpath for you?