expand diff.groovy to support 3rd parties

Similar to https://github.com/checkstyle/contribution/issues/340 , I would like diff.groovy to support non-specific 3rd party JARs.

I am splitting this off from sevntu just in-case sevntu's implementation has to be drastically different, or at the very least not conflict with it since it is slightly higher priority.

This could be done similar to our property file for repos. Give a repo location, build command, and before/after commits or branch names.