tonic-build: option to emit Arc<Self> as server receiver (#1352)

This implements an option to emit `Arc<Self>` instead of `&self` in
server traits. This enables implementor to reference `Self` data for
indefinite duration, which may be useful for streaming requests.

Fixes: #1351

tonic-health/src/generated/grpc.health.v1.rs

@@ -337,7 +337,9 @@ pub mod health_server {
                             request: tonic::Request<super::HealthCheckRequest>,
                         ) -> Self::Future {
                             let inner = Arc::clone(&self.0);
-                            let fut = async move { (*inner).check(request).await };
+                            let fut = async move {
+                                <T as Health>::check(&inner, request).await
+                            };
                             Box::pin(fut)
                         }
                     }
@@ -382,7 +384,9 @@ pub mod health_server {
                             request: tonic::Request<super::HealthCheckRequest>,
                         ) -> Self::Future {
                             let inner = Arc::clone(&self.0);
-                            let fut = async move { (*inner).watch(request).await };
+                            let fut = async move {
+                                <T as Health>::watch(&inner, request).await
+                            };
                             Box::pin(fut)
                         }
                     }