Add sp() output descriptor format for BIP352 Silent Payments

[craigraw]

This builds on and provides an alternative to #2026, which references an earlier discussion. Instead of modifying BIP352, a new output descriptor format is proposed in the style of BIPs 381-387.

Similar to #2026 key expressions starting with spscan and spspend are specified, but contain only version and key material information. The wallet birthday and additional labels are specified as optional additional arguments in the output descriptor format.

BIP352 authors: @josibake @RubenSomsen

Mail list discussion: https://groups.google.com/g/bitcoindev/c/bP6ktUyCOJI

[jonatack]

Looks sound and high quality for an initial draft. I don't see the BIP 2 specified mail list discussion for this proposal?

[craigraw]

Thanks for the reminder - I've sent the mailing list post and added the link in the BIP. Also, the spaces after commas in the descriptors have been removed to be consistent with BIP 380-387.

[pythcoiner]

Thanks for working on this!
Overall lgtm, the only comment I have is, maybe a way to specify a label range could be useful?

[craigraw]

Overall lgtm, the only comment I have is, maybe a way to specify a label range could be useful?

I'm not strongly opposed to this, but given the cost of scanning for each additional label I'm reluctant to add it - it may make a wallet synchronization trivially impractical in a non-obvious way simply by accidentally specifying a large range.

[pythcoiner]

Maybe with adding a note discouraging wallet to export a range?
I mainly see range usefull for exchanges that can have a wide range, but also for this special case I guess they also need to backup associate metada for each label, so the descriptor is not sufficient anyway...