fix: createdAt should be optional field

Problem to solve

RESPONSE FROM A CLIENT !!!

I use a generated api-client (based on the swagger file) and have not created a schema mit the DID of the ccm-engine, but called just schema_lookup_by_id() which raised an exception, because in the (json) data returned from ocm there is no createdAt field which is mandatory. I patched the swagger file and now it works, but that’s not a sustainable fix. I guess that in the ocm you query the ledger for the schema_id but don’t retrieve the createdAt value.