ci: Report a summary of changes on owlbot PRs

The mono-PR with all generated changes created by `owlbot` can be overwhelming to review, even infeasible given GitHub UI constraints. Since we already have individual `apidiff -incompatible` checks for each package that changes within a PR, we don't need to manually review for breakages.

That leaves reviewing _additions_ to the public surface. It would be nice if something generated a report of the elements _added_ to the public surface of each package for easier reviewing. This would also be nice in determining if some extraneous code was being generated when it shouldn't be.

Each package would be collapsable like so:

cloud.google.com/go/asset/apiv1

Compatible changes:

- Foo: added