BIP329: Add optional key origin property and expand truncation note

[craigraw]

Based on feedback received after BIP329 was initially published, this adds the following:

• An optional origin property containing key origin information which can be used to to disambiguate labels from different wallets contained in the same export
• A note on warning users if truncation is applied, along with a suggested maximum label length

[craigraw]

On further consideration, the origin property has been changed to reflect an abbreviated output descriptor (including key origin information but excluding actual keys, child path elements or a checksum). This is necessary to handle multiple script types and more complex wallets including multisig wallets.