release-plz release: parse any changelog (not only markdown or keep a changelog)

[MarcoIeni]

Motivations

Some people use a custom changelog template. E.g. see #1549

We use the parse_changelog library to parse the latest release from the changelog. So when we parse the last entry of a changelog (for example to attach release notes to github releases (do we also do the same for the PR body?) release-plz isn't aware of the [changelog] section of the configuration.

Solution

In theory release-plz could read the template we pass to git-cliff for changelog generation, and parse the latest changelog entry itself. Maybe use parse_changelog as a fallback.