use a trait object for the server, for dynamic dispatch

sync-server/src/api/mod.rs

@@ -1,2 +1,8 @@
+use crate::server::SyncServer;
+use std::sync::Arc;
+
 pub(crate) mod add_version;
 pub(crate) mod get_child_version;
+
+/// The type containing a reference to the SyncServer object in the Actix state.
+pub(crate) type ServerState = Arc<Box<dyn SyncServer>>;