feat: Add google.api.api_version annotation to echo.proto (#1484)

* feat: Add google.api.api_version annotation to echo.proto

* update the api_version to a nonsense value to refelect that it should be opaque to clients as per AIP

* Address review feedback

schema/google/showcase/v1beta1/echo.proto

@@ -43,6 +43,8 @@ service Echo {
   // This service is meant to only run locally on the port 7469 (keypad digits
   // for "show").
   option (google.api.default_host) = "localhost:7469";
+  // See https://github.com/aip-dev/google.aip.dev/pull/1331
+  option (google.api.api_version) = "v1_20240408";
 
   // This method simply echoes the request. This method showcases unary RPCs.
   rpc Echo(EchoRequest) returns (EchoResponse) {