Add support for BIP389 Multipath descriptors

[notmandatory]

Describe the enhancement

Once bitcoindevkit/bdk_wallet#11 is release support should be added to this project too.

Use case

This is useful for compactly defining an external and internal descriptor derivation path.

Additional context

https://github.com/bitcoin/bips/blob/master/bip-0389.mediawiki

[vadim-anfv]

@notmandatory The title and link reference BIP 383, but should be BIP 389. Might want to update.

[notmandatory]

@notmandatory The title and link reference BIP 383, but should be BIP 389. Might want to update.

Thanks for catching that! fixed.