Step runner version and changelog

In order to release significant changes (such as breaking changes) we need a changelog. And a way for users to see what additional changes are included in the version of Step Runner they are using.

This MR introduces a CHANGELOG.md which is initially indexed by date because we don't yet have release versions. And it's not really possible to predict what the commit SHA will be when making changes to the changelog. Once we create a release process we'll include semantic version in the changelog.

Because we are releasing every change to main we will only include breaking changes in the log. Other changes can be found by looking at the commit history of main. To facilitate this, Step Runner is now built with a short commit SHA which it prints upon startup along with a link to the changelog.