consider configurability for private key handling in debugging functionality

see issue #323

PR 359 replaces private keys with hardcoded signatures, resolving this issue. However, private keys are still sent whenever SignatureTemplate arguments are used in contract unlockers.

This is a feature, because this allows users to edit the bytecode in-place in the Bitauth IDE, while keeping the signatures valid. However, it can also be a drawback since it does expose private keys.

Ideally we would make this configurable.