camunda-engine should not expose gson dependency

Gson is shaded into a different package namespace in the camunda-engine jar. Gson itself should not be exposed as a transitive dependency.

Impact:

• Embedded engine users receive a Gson dependency
• We include Gson in the JBoss and Wildfly modules
• We include Gson in the standalone webapps

Implementation hints:

• Gson is currently in compile scope, but it should probably be in provided