Custom JAVA_OPTS

Previously, the user had no way to specify custom JAVA_OPTS to be used
when the application was running.  This change adds the ability for
the user to specify java.opts in system.properties and to have the
value added to the JAVA_OPTS contributed by the buildpack components.

[#49694697]
Merge 49694697-custom-java-opts to master

[Completes #49694697]