The idea is to clean the design of event transactions. Initially, we will support both repeated EventTransaction event_transaction and repeated bytes transaction in GossipEvent proto.

1. Inside TransactionBody add a new proto type - StateSignatureTransactionBody
2. Adapt GossipEvent proto to have repeated bytes transaction
3. Adapt event hashing logic - adding support for the new logic -> having transactions as bytes only
4. Adapt platform app tests
5. Adapt all APIs where hedera.app will receive the serialized bytes transactions and will return the system transactions back to the platform