Add mechanism for operator changelogs

Currently operators do not contain a changelo, much like how an RPM file can have a changelog. And while some operators might have release notes, release notes only provide a certain level of granularity / detail for changes.

It could be useful to incorporate a mechanism in OperatorHub that can display a changelog with the main changes from release to release.

So far one customer (Nokia) is asking for per-operator changelogs to help understand what has been changed from a major release to another and track where their reported issues have been implemented.