CPack: Add "CPack External" generator
This PR creates a new "CPack External" generator, which generates a .json file containing the metadata in CPackConfig.cmake.
A GitLab update will be applied on April 24th, between 8am and 10am, EST (UTC-05:00). This site may have intermittent delays during that time.
This PR creates a new "CPack External" generator, which generates a .json file containing the metadata in CPackConfig.cmake.