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>
Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>
Signed-off-by: Evgenii Stratonikov <evgeniy@nspcc.ru>