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/proto/test.proto

@@ -0,0 +1,12 @@
+syntax = "proto3";
+
+package test;
+
+service Test {
+  rpc TestRequest(SomeData) returns (SomeData);
+}
+
+message SomeData {
+  // include a bunch of data so there actually is something to compress
+  bytes data = 1;
+}