feat(metadata): expose MaintainerSince adoption timestamp

I'm looking at adding a warning to paru to alert users if an AUR package that was orphaned has been recently adopted to prompt the user that they should re-review a PKGBUILD. The RPC exposes the current Maintainer but never when ownership changed, so helpers would have to resort to local last-seen bookkeeping that misses fast adopt-then-push cycles and can't help on first install. Co-maintainers have push rights too, so they need the same signal.

This change aims to provide those datapoints for use downstream.