Add server interop tests

tonic-interop/src/lib.rs

@@ -1,3 +1,5 @@
+#![recursion_limit = "256"]
+
 pub mod client;
 pub mod server;
 
@@ -16,6 +18,13 @@ pub fn client_payload(size: usize) -> pb::Payload {
     }
 }
 
+pub fn server_payload(size: usize) -> pb::Payload {
+    pb::Payload {
+        r#type: default::Default::default(),
+        body: iter::repeat(0u8).take(size).collect(),
+    }
+}
+
 impl pb::ResponseParameters {
     fn with_size(size: i32) -> Self {
         pb::ResponseParameters {