Add support of upstream signed deltas format to v2021pre

Backported version of !57 (merged)

Starting v2020.7 libostree upstream support two versions of static deltas. Signed static delta has different binary format OSTREE_STATIC_DELTA_SIGNED_FORMAT and have an additional magic value as indicator of signed version.

This patch series allow to use signed and unsigned delta formats. In case if repository have option core.sign-verify-deltas=true only signed delta is allowed.