BIP329: add optional data fields, fix a JSON type

If the goal is solely to move labels between cooperating wallets, then previously defined values are the absolute minimum needed. However, wallet data exports can serve other purposes and many values associated with addresses, transactions and outputs are already on-hand for the wallet generating the export, and yet would be hard or impossible for importing tools to reconstruct. This PR proposes a bunch of optional fields that could be useful to users of this data. All are optional.

Specification