No private keys for mock signatures

[grarco]

We have introduced mock signature generation (and verification) for dry runs in #4832. To minimize code duplication in the SDK we used the same interface of sign_wrapper for mock_sign_wrapper and the same interface of sign_raw for mock. Both these mock functions still require a private key as a parameter, which makes little sense given that they won't produce an actual signature. This causes problems with Namadillo since it needs to dry-run transactions (with mock signatures) without interacting with the extension. We should change these mock functions to require public keys instead of private ones.

cc: @mateuszjasiuk