Add an explicit `TxBuilder` method to enable non-ALL sighashes

BDK signs using the sighash specified in the PSBT (or `ALL` if it's not provided). It's currently up to the user of the library to check that the PSBT they are signing doesn't have a dangerous sighash, but to avoid accidents it's probably better to have this check internally and force the user to explicitly opt-in to use non-ALL sighashes.