Parsing PoolCertificate in Conway fails

**Describe the bug**
Trying to process a transaction with a PoolCertificate created in Conway era fails since pool_owners is a Set instead of List

**To Reproduce**
Create PoolCertificate using cardano-cli in conway era, then try to parse the CBOR hex.

**Expected behavior**
Should be able to parse certificates from all eras


**Additional context**
Incoming pull request with the fix
