config: enable full for tokio-util in the playground (tokio-rs#6818)

Author: sarsko

tokio-util/Cargo.toml

@@ -65,3 +65,6 @@ rustdoc-args = ["--cfg", "docsrs", "--cfg", "tokio_unstable"]
 # it's necessary to _also_ pass `--cfg tokio_unstable` to rustc, or else
 # dependencies will not be enabled, and the docs build will fail.
 rustc-args = ["--cfg", "docsrs", "--cfg", "tokio_unstable"]
+
+[package.metadata.playground]
+features = ["full"]