lib/deltas: Add inline signature for static-delta superblock

lib/deltas: Add inline signature for static-delta superblock

While the commits contained in the single static-delta file are signed so
we can check them and operate on trusted data, the superblock isn't signed
in any way, so it end up operating on untrusted data to:
 1. actually find where the trusted data is, and
 2. check whether the update is fit for the current device by looking at
    the collection id stored in the metadata

This commit generates signatures of all static data, and concatenate them
to the existing static delta format, i.e. as a GVariant layout `a{sv}ay`
where
 - a{sv}: signatures
 - ay: existing delta variant

Signed-off-by: Frédéric Danis <frederic.danis@collabora.com>