dissect: add support for verity-protected bare filesystems via mountfsd (#39325)

Needed to implement support for RootHashSignature=/RootVerity=/RootHash=
and friends when going through mountfsd, for example with user units,
so that system and user units provide the same features at the same
level

diff --cc src/libsystemd/sd-json/json-util.h
Simple merge

diff --cc src/shared/dissect-image.h
Simple merge