Add evaluation scenarios for placeholder_p2pkh inputs & pass pkh/pubkey/sig variables instead of passing a private key

[mr-zwets]

Currently in the libauth debugging tooling PrivateKeys are included in the overwrites, this could potentially expose real private keys used to hold funds on mainnet.

Instead we should consider including the signature directly so the PrivateKey is not exposed over the network or potentially shared with others. If the template is a valid transaction, the signature could still potentially be used to spend the input depending on the hashtype and the status of the utxo.

[rkalis]

We realised that making this change also would make P2PKH-only transactions properly debuggable. Next time we want to add evaluation scenarios for P2PKH inputs in the libauth template generation. As a part of this we also want to change the lock/unlock scripts to use separate variables for the pkh/pubkey/signature, rather than deriving them all from the same private key.