A Jcat file can be used to store GPG, PKCS-7 and SHA-256 checksums for multiple
files. This allows us to sign a firmware or metadata multiple times (perhaps
by the OEM and also then the LVFS) which further decentralizes the trust model
of the LVFS.
The Jcat format was chosen as the Microsoft catalog format is nonfree and not
documented. We also don't want to modify an existing .cat file created from WU
as this may make it unsuitable to use on Windows.
More information can be found here: https://github.com/hughsie/libjcat
Replace fu_common_cab_build_silo() with an actual GObject that can hold parsing
state. This cleans up the code a lot, and means we can add additional
functionality in the future without breaking ABI or API.
The long term plan is to verify the metadata and payload signatures when
parsing FuCabinet, rather than much later in _check_requirements().
This of course requires passing in a keyring context (which we don't yet have)
and would mean we can stop setting the various confusing 'fwupd::ReleaseBlob'
XbNode extra data.
No logic changes for now, just a lot of moving things into sane places.
