Include output index in Onchain Payment Kind

[rolznz]

For listing on-chain transactions in LDK-Node only the Txid is exposed. Would it also be possible to expose the output index? e.g. If you receive a payment from an exchange or some other payment with many outputs and only one is yours, if you open up the TXID in a transaction explorer, it can be difficult to find your output

[tnull]

Hmm, this seems closely related to (if not even duplicate of) #684. As stated there, I'm not sure we have a straightforward way to do this right now, but will explore the possibilities.