Extract CHANGELOG entries generation (and CHANGELOG renderer) to a gem

Description

We have automated CHANGELOG generation using the bin/changelog command, and fixed the conflicts by changing the CHANGELOG.md only at the release.

But this is only available in the gitlab main repository (CE and EE). There are other projects where we have the same issues and replicating the code is not the best alternative.

Proposal

We should extract the code from gitlab main repository an release-tools to a Gem, that we can reuse in other projects.

We could either base our code on existing one or consider https://gitlab.com/basalt/keepachangelog which works very similar.

Links / references

• https://docs.gitlab.com/ce/development/changelog.html#how-to-generate-a-changelog-entry
• https://gitlab.com/gitlab-org/release-tools

cc @stanhu @DouweM @rspeicher @rymai @nick.thomas @smcgivern