Need access to process variables tags at FormRendererBase (kie-server-services-jbpm-ui)

The ProcessVariable tags such as required and read-only need to be available in `org.kie.server.services.jbpm.ui.FormRendererBase` to apply the validation in the forms.
This will require some changes at JBPM services.

being pinged about this. Access variable tags at this point
https://github.com/kiegroup/droolsjbpm-integration/blob/3f7b61d15fa28160e58a4520658d486487ccad2e/kie-server-parent/kie-server-services/kie-server-services-jbpm-ui/src/main/java/org/kie/server/services/jbpm/ui/FormRendererBase.java#L249