Support multi-path descriptors

[nochiel]

Sparrow Wallet generates "multi-path patterns" in its output descriptors. However, these patterns are not part of the bitcoin core descriptor specification. As such they are also not supported by BDK upon which Spotbit depends.

BIP88 (Hierarchical Deterministic Path Templates) has been proposed.

Spotbit needs to be able to parse such descriptors to obtain a change address descriptor that can be used to generate a BDK wallet so that a beancount report can be generated for relevant transactions.

References

• I've written an initial implementation of the parser in 8043aee which needs to be improved and tested.
• Sparrow's parser