Returns the vendor of an attachment envelope: the object's 'vendor'
read by subject extraction (the reference's
extract_object_for_predicate), so a salted or otherwise annotated vendor
is read too; an empty vendor is a vendor.
EnvelopeError with code InvalidAttachment when the envelope is
not an assertion; NonexistentPredicate / AmbiguousPredicate when
there is not exactly one vendor; Cbor (dcbor error: <Display>) when
it is not text.
Returns the vendor of an attachment envelope: the object's
'vendor'read by subject extraction (the reference'sextract_object_for_predicate), so a salted or otherwise annotated vendor is read too; an empty vendor is a vendor.