fix: violations of elided_named_lifetimes (#4936)

Author: WenyXu

src/servers/src/mysql/handler.rs

@@ -249,7 +249,7 @@ impl<W: AsyncWrite + Send + Sync + Unpin> AsyncMysqlShim<W> for MysqlInstanceShi
         self.auth_plugin()
     }
 
-    async fn auth_plugin_for_username(&self, _user: &[u8]) -> &str {
+    async fn auth_plugin_for_username<'a, 'user>(&'a self, _user: &'user [u8]) -> &'a str {
         self.auth_plugin()
     }