[GSS] (6.4.z) back port functionality of findSubProcessInstances to kie-server 6.x

Currently, we have java remote api that supports the method
List<ProcessInstanceLog> findSubProcessInstances(long processInstanceId)
which returns a list of ProcessInstanceLog instances of sub processes based on their parent processInstance id.

However, this call is only available for business-central in bpms 6.x and kie-server 7.x [1].

Can we back port this functionality to the rest api for kie-server in 6.x?

[1] https://github.com/kiegroup/droolsjbpm-integration/blob/master/kie-server-parent/kie-server-remote/kie-server-rest/kie-server-rest-jbpm/src/main/java/org/kie/server/remote/rest/jbpm/ProcessResource.java#L746