Parse multiple conventional commit messages

[chingor13]

Sometimes we get autosynth PRs that include multiple changes (grouped together from a single proto push).

Currently, we try to slam as much information into the main commit title so that we can indicate everything included.

It would be nice to be able to add additional "commit messages" that would show up in the release notes

[chingor13]

I propose something like the following in the commit body somewhere:

BEGIN_RELEASE_NOTES
feat: foo bar
docs: asdf qwer
END_RELEASE_NOTES

Both these messages would appear in the release notes and affect the semantic versioning, but would link to the same real commit/pull request.