DMN FEEL GWT Showcase only on explicit GWT properties

To avoid having long time of build for a Showcase (GWT FEEL Showcase) which does contain some test/integration tests for checking FEEL expression evaluation after being transpiled to JS, the Maven module will be moved into a separate profile which is not active by default.

To activate the profile, either specify with usual -PfullProfile option, either explicit set the -Dfull property.

This solution can be extended equivalently when J2CL will be used in place of GWT per this comment https://github.com/kiegroup/drools/pull/3678#issuecomment-862168817