feat: expose derived_address helper on Descriptor

[reez]

Description

exposing a helper to derive an address from a descriptor without building a wallet

follows miniscript’s recommended at_derivation_index → derived_descriptor flow to avoid panicy edge cases with unsupported descriptor https://docs.rs/miniscript/12.3.5/src/miniscript/descriptor/mod.rs.html#886

Notes to the reviewers

tests covering both the happy path and multipath error mapping

Documentation

The helper includes these

• bdk_wallet

  https://docs.rs/bdk_wallet/latest/bdk_wallet/descriptor/enum.Descriptor.html#method.at_derivation_index
  https://docs.rs/bdk_wallet/latest/bdk_wallet/descriptor/enum.Descriptor.html#method.derived_descriptor-1

• bitcoin

  https://docs.rs/bitcoin/0.32.7/bitcoin/address/struct.Address.html#method.from_script

• uniffi

• Other:

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing
• I've added a changelog in the next release tracking issue (see example)
• I've linked the relevant upstream docs or specs above

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR

[reez]

Need to add changelog once this comes out of Draft

[reez]

CI passes except for Android but I think that's a known thing atm

[thunderbiscuit]

I'm not sure of the exact difference between the two approaches (also the bdk_wallet type is a wrapper around the rust-bitcoin type and so has new methods I wasn't aware of initially), so let me know if this is wrong somehow but I was able to do away with the use of dervied_descriptor entirely and simplify quite a bit (mind you with handling the errors it will get bigger):

Currently:

let secp = Secp256k1::verification_only();
let derived_descriptor = self
    .extended_descriptor
    .at_derivation_index(index)
    .and_then(|desc| desc.derived_descriptor(&secp))
    .map_err(|error| match error {
        ConversionError::HardenedChild => DescriptorError::HardenedDerivationXpub,
        ConversionError::MultiKey => DescriptorError::MultiPath,
    })?;

let script_pubkey = derived_descriptor.script_pubkey();
let address =
    Address::from_script(Arc::new(Script(script_pubkey)), network).map_err(|error| {
        DescriptorError::Miniscript {
            error_message: error.to_string(),
        }
    })?;

Ok(Arc::new(address)

Can be rewritten as:

let definite_descriptor = self.extended_descriptor.at_derivation_index(index).unwrap();
let address = definite_descriptor.address(network).unwrap();
        
Ok(Arc::new(address.into()))

Let me know if I'm missing something.