[Feature Request] Add support for custom PIN length

### Reasoning or why should it be changed/implemented?
A fixed PIN length is undesirable as users may want a different length for a plethora of reasons, a key one that comes to mind being that a user might want the PIN to be the same as the one they use to unlock their iPhone, which is usually 6 numbers long.
### iOS version
iOS 12.5.1
### App version
v3.1.0.7