Libraries and tools for v3.4: add Scala reference (#286)

* Update libraries and tools

Include mingchuno/etcd4s library for Scala with V3 support and watch using Akka Stream.

* Tweaked the description

Co-authored-by: Patrice Chalin <chalin@users.noreply.github.com>

content/en/docs/v3.4/integrations.md

@@ -41,6 +41,7 @@ description: A listing of etcd tools and client libraries
 
 - [maciej/etcd-client](https://github.com/maciej/etcd-client) - Supports v2. Akka HTTP-based fully async client
 - [eiipii/etcdhttpclient](https://bitbucket.org/eiipii/etcdhttpclient) - Supports v2. Async HTTP client based on Netty and Scala Futures.
+- [mingchuno/etcd4s](https://github.com/mingchuno/etcd4s) - Supports v3 using gRPC with optional Akka Stream support.
 
 **Perl libraries**