[signer] Add an option to explicitly allow using non-`ALL` sighashes

[signer] Add an option to explicitly allow using non-`ALL` sighashes

Instead of blindly using the `sighash_type` set in a psbt input, we
now only sign `SIGHASH_ALL` inputs by default, and require the user to
explicitly opt-in to using other sighashes if they desire to do so.

Fixes #350