Implement `String` / `FromString` method pair in all levels of enum
definitions. From now `String()` returns canonical protojson-compatible
values.
Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>
Extended ACL filters and object search filters use strings to specify
well-known object headers, such as owner id, object id, etc. To unify
these constant they've been moved into object package, because they
specifying object related headers.
This should be noted in nspcc-dev/neofs-spec#12
Signed-off-by: Alex Vanin <alexey@nspcc.ru>
