Change the add method to also accept bip389 descriptors (multipath descriptor) (in addition to the already-supported external/internal format). This format is supported natively in Sparrow.

We can optionally fix this issue for the BDK team bitcoindevkit/bdk_wallet#11

Use this library to implement upstream in BDK, then Smaug, or just do it in Smaug and don't worry about upstreaming it into BDK.