Support Multipath Descriptors in `chain`

[tvpeter]

Describe the enhancement

The current implementation of chain supports descriptors that represent only a single keychain path. With the introduction of multipath descriptors (and its implementation in bdk_wallet 2.1.0), a single descriptor can represent multiple keychains, both external and internal paths.

Under the current architecture, providing a multipath descriptor to several core components results in MultiKey errors or panics because the code attempts to derive scripts without specifying a path index.

Use case

In bdk-cli, several wallet features such as full_scan, sync, public_descriptor, policies panics and fails with MultiKey error.
See bdk-cli#238

Impact

• Blocking production usage

Are you using BDK in a production project?

• Yes
• No
• Not yet, but planning to

Which backend(s) are relevant (if any)?

• Electrum
• Esplora
• Bitcoin Core RPC
• None / not backend-related (e.g. bdk_chain, bdk_core)
• Other (please specify): ____