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

tests/use_arc_self/src/lib.rs

@@ -0,0 +1,20 @@
+#![allow(unused_imports)]
+
+use futures::{Stream, StreamExt};
+use std::sync::Arc;
+use tonic::{Request, Response, Status};
+
+tonic::include_proto!("test");
+
+#[derive(Debug, Default)]
+struct Svc;
+
+#[tonic::async_trait]
+impl test_server::Test for Svc {
+    async fn test_request(
+        self: Arc<Self>,
+        req: Request<SomeData>,
+    ) -> Result<Response<SomeData>, Status> {
+        Ok(Response::new(req.into_inner()))
+    }
+}