feat: collect pubspec versions on release for comparison with patch

[eseidel]

It's somewhat common for users to not check in their pubspec.lock, and then end up with unexpected (sometimes native) changes to their packages.

https://docs.shorebird.dev/troubleshooting/#i-see-a-your-app-contains-native-changes-warning-when-running-shorebird-patch-even-though-i-havent-changed-swiftobjective-ckotlinjava-code

It might help if when we show a native warning, if we could show what pubspec versions have changed since the release.

Instead of saving the whole pubspec.lock (which might have paths in it), we could just save a map of package to version to be maximally privacy respecting.