sidedata: add a `decode_files_sidedata` function Right now the function mostly gather existing code to build a consistent object. However having this function allow us prepare all user code independently from the actual side data format change (and associated encoding/decoding changes) Strictly speaking, we could not need to passe the sidedata explicitly since we have access to it though the `changelogrevision` object. However, the short term goal is to drop that first parameter to only pass the sidedata binaries.