Spending Silent Payment outputs with PSBTs.

Would add a PSBT field holding the extra key detail a signer needs to spend coins received through silent payments.

Coins received by silent payments sit at Taproot addresses whose keys are adjusted in a way normal Taproot signing does not expect. BIP 375 covers sending; this BIP covers spending, by letting a PSBT, the file passed around for signing, carry the adjustment the signing device needs.