sidedata: add a `decode_files_sidedata` function...
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.
