pcrextend-util: add helpers for measuring roothash/signature of Verity volumes

pcrextend-util: add helpers for measuring roothash/signature of Verity volumes

This adds infrastructure for measuring Verity root hashes from
userspace, along with he issuer/serial of the signatures used to unlock
them.

We measure the triplet of volume name, root hash and issuer/serial. if
confext/sysext use different signing keys then this ensures the event
log carry information about the type of image measures.