DescriptorSecretKey.extend() method requires a wildcard be applied to the root extended key

[thunderbiscuit]

Our current implementation of the .extend() method carries over the wildcard choice made initially by the DescriptorSecretKey type. This means that if the DescriptorSecretKey was created without a wildcard, there is no way to extend it with a wildcard.

Example:

val rootDescriptorSecretKey = DescriptorSecretKey.fromString(
    "tprv8ghw3FWfWTeLCEXcr8f8Q8Lz4QPCELYv3jhBXjAm7XagA6R5hreeWLTJeLBfMj7Ni6Q3PdV1o8NbvNBHE59W97EkRJSU4JkvTQjaNUmQubE"
)
val keysAtNewPath = rootDescriptorSecretKey.extend(DerivationPath("m/1212"))
val descriptorTest1 = Descriptor(
    descriptor = "wpkh(${keysAtNewPath.asString()})",
    network = Network.TESTNET
)
println("Issue #324 descriptor is ${descriptorTest1.asStringPrivate()}")
// Issue #324 descriptor is wpkh(tprv8ghw3FWfWTeLCEXcr8f8Q8Lz4QPCELYv3jhBXjAm7XagA6R5hreeWLTJeLBfMj7Ni6Q3PdV1o8NbvNBHE59W97EkRJSU4JkvTQjaNUmQubE/1212)#8qetj2j9

We cannot apply the wildcard to the DerivationPath type, and so the only way to have a wilcard for this path is to create the root key with a wild card in the first place:

val rootDescriptorSecretKey = DescriptorSecretKey.fromString(
    "tprv8ghw3FWfWTeLCEXcr8f8Q8Lz4QPCELYv3jhBXjAm7XagA6R5hreeWLTJeLBfMj7Ni6Q3PdV1o8NbvNBHE59W97EkRJSU4JkvTQjaNUmQubE/*"
)
val keysAtNewPath = rootDescriptorSecretKey.extend(DerivationPath("m/1212"))
val descriptorTest1 = Descriptor(
    descriptor = "wpkh(${keysAtNewPath.asString()})",
    network = Network.TESTNET
)
println("Issue #324 descriptor is ${descriptorTest1.asStringPrivate()}")

// Issue #324 descriptor is wpkh(tprv8ghw3FWfWTeLCEXcr8f8Q8Lz4QPCELYv3jhBXjAm7XagA6R5hreeWLTJeLBfMj7Ni6Q3PdV1o8NbvNBHE59W97EkRJSU4JkvTQjaNUmQubE/1212/*)#lgncmkah

This little quirk of the API comes from the fact that in the FFI we extend the DescriptorSecretKey "directly", whereas the .extend() method in rust-bitcoin belongs to the DerivationPath type. This little bug (not really a bug but just a weird behaviour?) doesn't have an easy answer, because it might involve reshaping our API and the way we extend keys with custom paths.

Opening this issue for discussion.

[thunderbiscuit]

Adding an argument for the wildcard sounds like a good way to adjust and allow for the functionality we need.