Generate proof to unlock NFT for accepted ownable

[jasny]

The accept method in the ownable service should return a cryptographic signature (aka proof) that can be used to unlock the NFT.

To test, mint an NFT (see #2) in the locked state.

Set the NFT information when instantiating the ownable.

{
  "@context": "instantiate_msg.json",
  "ownable_id": "...",
  "package": "...",
  "nft": {
    "network": "eip155:5",
    "address": "...",
    "id": 1
  }
},
{
  "@context": "execute_msg.json",
  "msg": {
    "lock": {}
  }
}

After applying the events, the state will include the NFT information set during instantiation. The ethers service will allow you to connect to the contract (remove eip155 from network to get the ethereum chain id). Get the challenge for the id.

Use ethers to sign the challenge, which will be the proof that should be returned.

Check (in code or via etherscan) to make sure you can unlock the NFT with that proof.

[jasny]

@bekauz Can you make sure that setting the NFT props works correctly in the car Ownable.