serializers: Prepare rename of 'identifiers_enum' to 'swhids_enum'.
This will be done in three steps to avoid any disruption:
- (this step) add support for decoding the new name, but keep encoding as the old one
- start encoding as the new name
- remove support for decoding the old name
Depends on !729 (closed).
Migrated from D6433 (view on Phabricator)