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

[afilini]

Description

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

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

New Features:

• I've added tests for the new feature
• I've added docs for the new feature
• I've updated CHANGELOG.md

[LLFourn]

ACK 881ca8d. I guess what you really want is to make sure that you are not signing any of your inputs with an unusual sighash (regardless of what others do). I don't think we need to account for this though since you are likely going to have to do a lot of manual verification work anyway in more exotic scenarios.

[notmandatory]

utACK 881ca8d. Test is easy to follow and covers expected cases.

[afilini]

I guess what you really want is to make sure that you are not signing any of your inputs with an unusual sighash (regardless of what others do). I don't think we need to account for this though since you are likely going to have to do a lot of manual verification work anyway in more exotic scenarios.

Yeah I tried to do that initially but it was a mess. If somebody needs that kind of "precision" they can just always opt-in on the BDK side and implement the check themselves before calling sign()