Add docs + test on required use of xpub for multipath constructor

[thunderbiscuit]

This PR makes explicit a limitation of the Wallet::create_from_two_path_descriptor method, in that you must use public descriptors to build the wallet using this constructor, meaning you can only build watch-only wallets.

I also added a test to showcase this and the error returned.

See my comment here for more:

From what I can tell the issue is that miniscript cannot parse an xprv (I saw a comment from Sanket about this somewhere but now I can't find it anymore) if the multipath contains hardened derivation paths, and therefore miniscipt cannot ensure the xprv will be parsable into public keys in all cases? Something like that. But our arguments for bdk wallet constructors require the IntoWalletDescriptor trait:

pub trait IntoWalletDescriptor {
    /// Convert to wallet descriptor
    fn into_wallet_descriptor(
        self,
        secp: &SecpCtx,
        network: Network,
    ) -> Result<(ExtendedDescriptor, KeyMap), DescriptorError>;
}

pub type ExtendedDescriptor = Descriptor<DescriptorPublicKey>;

In other words, upon creation, the wallet will always try to parse the descriptor string into its public form, and here since it cannot be done by miniscript we fail to build.

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[coveralls]

Pull Request Test Coverage Report for Build 17476589408

Details

• 5 of 5 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.01%) to 84.922%

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Totals
Change from base Build 17330390829:	0.01%
Covered Lines:	6674
Relevant Lines:	7859

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💛 - Coveralls

[ValuedMammal]

ACK e647ed2

Further, the presence of hardened steps doesn't seem to make a difference, as this test shows. It's important to document the limitation as this PR does but to be honest I haven't found a good explanation for why it exists.

[oleonardolima]

utACK e647ed2

[thunderbiscuit]

to be honest I haven't found a good explanation for why it exists

Yeah it's fuzzy for me too but I think the answer probably is in this video if I remember correctly. He goes over the ideas and tricks that make extended keys able to create children keys, and how this ability is broken by the "hardening" process.

It's maybe that because you can't ensure that some of the multipath paths are not hardened, miniscript just errors out on all multipath xprv. But also, could you not just error out if one or more of those paths are hardened instead of in all cases? I might be thinking about this wrong.