feat. request: sign PSBTs with xprivs/descriptors

sth like here https://github.com/BlockchainCommons/sweeptool-cli/blob/master/docs/MANUAL.md#signing-a-psbt

Start with the linked Sweeptool manual's PSBT signing section and compare its xpriv and descriptor inputs with keytool-cli's current command surface. Define the supported signing inputs and a clear completion check for signing a PSBT, then identify the relevant CLI entry point and add focused coverage.

索引モデルが issue の本文から書いたものです。