Add Github workflow to ensure changelogs are added

[arminru]

The changelog in the spec repo has to be updated manually for each PR but sometimes PR authors forget to do so. Relying on reviewers to catch this is error-prone. In other OTel repos we use Github workflows to verify that the respective CHANGELOG.md was updated in PRs. For minor/editorial edits this check can be bypassed by applying a certain label.

Should we add the same for the spec repo as well?

See https://github.com/open-telemetry/opentelemetry-collector-contrib/blob/main/.github/workflows/changelog.yml for an example.

[arminru]

We could also add the same for the https://github.com/open-telemetry/opentelemetry-specification/blob/main/spec-compliance-matrix.md to ensure rows are added for new requirements.