Add CI to generate CHANGELOG

Many of the other SDK libraries have a GH action that runs as part of CI to generate CHANGELOG.md for version updates/releases; for example: https://github.com/contentauth/c2pa-node/blob/main/CHANGELOG.md generated by https://github.com/changesets/action.

This repo should have something similar, to provide an easy reference for changes in a given release. 