Transaction: Keep metadata as GBytes
Transaction: Keep metadata as GBytes

This makes it more easy to expose these without constantly
duplicating the memory.

Closes: #1799
Approved by: alexlarsson

matthiasclasen/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,flatpak/flatpak,matthiasclasen/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,matthiasclasen/flatpak