tools: add rust-etcd to the list of libraries.
- [efrecon/etcd-tcl](https://github.com/efrecon/etcd-tcl) - Supports v2, except wait.
+**Rust libraries**
+
+- [jimmycuadra/rust-etcd](https://github.com/jimmycuadra/rust-etcd) - Supports v2
+
**Gradle Plugins**
- [gradle-etcd-rest-plugin](https://github.com/cdancy/gradle-etcd-rest-plugin) - Supports v2
