Proposal: Replace OS packages with UKIs or simply OS package format?

This is the direction we're planning to go in eventually, exactly how and when is unclear. There's a start of a proposal that discusses the signature parts of this in relation to Sigsum by @zaolin, see https://git.glasklar.is/system-transparency/project/documentation/-/blob/main/proposals/2023-12-01-ospkg-uki-transition.md.