Feature: Expose add_global_xpubs

[andreasgriffin]

Describe the enhancement

Please expose add_global_xpubs from https://github.com/bitcoindevkit/bdk/blob/d99b3ef4b449a5c7b8facf04db3cb9be829a0ac1/crates/wallet/src/wallet/tx_builder.rs#L530

Use case
Some hardware wallets, need the xpubs field filled (e.g. bitbox02 ) to be able to sign multisig psbts.