RESTWorkItemHandler doesn't support proxy

It seems that the default rest handler:
https://github.com/kiegroup/jbpm/blob/master/jbpm-workitems/jbpm-workitems-rest/src/main/java/org/jbpm/process/workitem/rest/RESTWorkItemHandler.java

does not support proxy.

Traditionally, JVM usually respects following proxy properties:
https://stackoverflow.com/questions/120797/how-do-i-set-the-proxy-to-be-used-by-the-jvm
For example:

http.proxyHost
http.proxyPort

Our handler does ignores these.

The reason is because we are using Apache HttpClient - it has the ability to support these system properties as well, but this ability is DISABLED by default.

Please see for more detail:

https://github.com/zzphamvanthanhzz/httpclient-4.5.2-java/blob/master/httpclient/src/main/java/org/apache/http/impl/client/HttpClientBuilder.java#L125

I am suggesting to add a new RESTWorkItemHandler constructor which accepts boolean flag which would allow to enable these system properties.

If this boolean flag would be true, then HTTPClient would be built with useSystemProperties(); call

We have actually tested useSystemProperties() in a real proxy environment, and we have verified that it works as per our expectations, see:

https://github.com/agiertli/ProxyEnabledRESTWIH/tree/master/src/main/java/org/redhat/services/rest

We believe that this almost trivial enhancement can bring a lot of value and power which Apache HTTPClient gives you.

I was also contemplating submitting a PR into community, but didn't figure out how to design a unit test, so I' rather leave it to the core maintaners.