Add optional fields in tx output hierarchy. Add Plutus V4 fields in proto definition.

[carbolymer]

This PR adds two changes:

1. Marks optional fields in TxOutput types hierarchy with optional modifier. This results in cleaner code generated, for example in Haskell, where you can explicitly say that the field is not present. Note that this is not changing the wire format: https://protobuf.dev/programming-guides/proto3/#field-labels
2. Adds Plutus V4 fields. Those fields are not in use yet in current cardano era, but they are already present in cardano-node Haskell code.