Allow the client to only sign for a subset of key placeholders

[bigspider]

Currently, the app only uses the PSBT_IN_BIP32_DERIVATION/PSBT_IN_TAP_BIP32_DERIVATION to figure out the right change/address_index pair for each input. However, there is no way of explicitly instructing not to sign for some placeholders; this is a common need for miniscript-based wallets like Liana, where multiple spending paths might be controlled by the same device (with different derivations paths).

Rather than modifying the protocol, the simplest way might be to not sign with a placeholder if the corresponding PSBT_GLOBAL_XPUB is missing in the PSBT.