actions: add action to validate the CHANGELOG

[sam-berning]

Issue number:

Closes #

Description of changes:

Ports changes for bottlerocket-os/bottlerocket-kernel-kit#13 into this repo.

Testing done:

Tested script locally, against current CHANGELOG:

$ ./tools/validate-changelog.sh 
CHANGELOG validation PASSED!

With a "v" removed from one of the version headers:

$ ./tools/validate-changelog.sh
1d0
< 1:# 5.2.0 (2024-12-20)
CHANGELOG validation FAILED! Headers must match the regex '^# v[0-9]\+\.[0-9]\+\.[0-9]\+.'

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.

[larvacea]

This gives me the validation I crave.