This adds support for [key origin](https://gist.github.com/sipa/e3d23d498c430bb601c5bca83523fa82#key-origin-identification) information to the descriptor parser, and exposes the resulting key path information through `FlatSigningProvider`.

There is no observable functionality from this right now, except having the `scantxoutset` RPC accept descriptors that include key origin information.

Longer term this feature helps with a potential descriptors-based walletless PSBT updater, or for importing hardware wallet xpubs (once the wallet can import descriptors).

ninja
ninja check
ninja check-functional