Multipath descriptors support

[darosior]

Multipath descriptors were proposed as a way to optimize the descriptor backup for the common case of having two derivation chains (one for receiving addresses, one for change).

A BIP was proposed a couple months ago: bitcoin/bips#1354. There is an implementation in Bitcoin Core at bitcoin/bitcoin#22838.

It was previously discussed at length in bitcoin/bitcoin#17190.

I think it would be helpful for us to support them. What do you think?

[apoelstra]

Yes, I do think we should support them.

[darosior]

I'm working on something. It's a quite invasive change, i'm adding a new variant of DescriptorPublicKey that contains a DescriptorMultiXKey. I think i'll have to change some interfaces as some assumptions necessarily don't hold anymore, for instance

rust-miniscript/src/descriptor/key.rs

Lines 406 to 411 in 9ea094c

	/// Full path, from the master key
	///
	/// For wildcard keys this will return the path up to the wildcard, so you
	/// can get full paths by appending one additional derivation step, according
	/// to the wildcard type (hardened or normal)
	pub fn full_derivation_path(&self) -> bip32::DerivationPath {

[darosior]

I've opened a WIP pull request to discuss the approach. #470