[Feature] CheatCode::set_signer

[d0cd]

🚀 Feature

A cheat code to set the signer via private key could be useful.
For example, CheatCode::set_signer(<PRIVATE_KEY>).
An open question is how should the private key be passed in. Leo doesn't have strings, but supporting them in this limited context (where it does not need to be compiled) could be okay.
Alternatively, we could support CheatCode::set_signer_address but that wouldn't have the same general properties as the original option.