How to get Workitems from a given plan using RTC client libraries ?

There is no public API available for planning. There is internal API that you should not use. There has been some internal API for it in the past. https://rsjazz.wordpress.com/2015/06/11/creating-plans-with-the-plain-java-client-api/ shows some of it. since planning is removed from the Eclipse client that is probably no longer maintained and should not be used (or knowng that it might break at some point).

Note that a "plan" is just a dynamic work item query, and you can use work item queries to get the same work items.