secp256r1 sign cheat code

[wilsoncusack]

Component

Forge

Describe the feature you would like

With the finalization of RIP-7212 and the progress of EIP-7212, more forge users will want a way to test signing using the secp256r1 curve.

I propose a new cheat code

function signP256(uint256 privateKey, bytes32 digest) external returns (bytes32 r, bytes32 s);

Additional context

P256 is the general abbreviation for this signing curve, though maybe a more verbose name would be more clear, given Ethereum uses secp256k1. Open to ideas.