Add abstract storage support to digest_filename()
Add abstract storage support to digest_filename()

Add an optional 'storage_backend' argument to digest_filename() which
expects an object implementing
securesystemslib.storage.StorageBackendInterface and defaults to None

Signed-off-by: Joshua Lock <edc229f50a11a11cdece0270af6320fb299dca1e@vmware.com>

secure-systems-lab/securesystemslib,secure-systems-lab/securesystemslib