[Feature] Create Changelog validator script.

[glennsarti]

Creating the changelog is a manual process, we should create a quick validation script that finds any missing Pull Requests that have been merged, but has a missing Changelog entry. To assist in the release process.

Note - Automating the changelog is not a great idea, verification is.

[jpogran]

Closing as we can get more value from improving our processes or having been controls on pull requests merges than spending time on tooling