Document how to set `https` proxy for Azure Event Hub input plugin

[andsel]

Azure Event Hub input plugin uses a client (deprecated) that leverages the HTTP client provided by the JDK. Such client can be configured to use https proxy. Update documentation, in plugin or on Logstash trouble shooting section maybe, to explain hot to configure such proxy.

---

Pre-existing knowledge

From this guide on the new AzureSDK (the plugin is using a deprecated library!) seems that all Azure java client libraries are able to respect the Java proxy environment variables. In particular they need the user to set:

-Djava.net.useSystemProxies=true
-Dhttps.proxyHost=<to fill>
-Dhttps.proxyPort=<to fill>
-Dhttp.proxyUser=
-Dhttp.proxyPassword=