gRPC: Make optional fields optional in proto definition

[carbolymer]

Update proto definitions to make fields optional:

(partially from #885 remarks)

Additional tasks:

• redeemer optional in TxInput (from: Add cardano-rpc package #885 (comment))
• assets, datum and script optional in TxOutput (from: Add cardano-rpc package #885 (comment) )
• payload and original_cbor optional in Datum

[carbolymer]

To be fixed by: utxorpc/spec#177