feat: extend signer to sign arbitrary message hashes

[taureau75]

This is a pull request to allow wallets to sign messages without broadcasting them.

Ethereum had a lot of issues with this, so it will likely require some debate and the establishment of a standard on how messages are signed. See EIP 712 and EIP 191.

https://eips.ethereum.org/EIPS/eip-191
https://eips.ethereum.org/EIPS/eip-712

I know there's a lot of security issues around it, so I don't expect it to get approved without a lot of discussion and additional code, but I wanted to get the ball rolling by putting up some code to start.

[taureau75]

For Zigzag, we sign orders off-chain and send them on-chain when they are matched.

Without the ability to sign messages through an external Signer, we can't support external wallets like Argent browser extension.

[taureau75]

The security issue is that if you allow arbitrary messages to be signed, a malicious app can present a user with a valid Starknet transaction to sign.

Since the garbled message won't look like a valid transaction, a user might sign it and sign their funds away.

[0xs34n]

Thanks for starting this PR, let's continue the discussion here:

argentlabs/argent-x#14

[0xs34n]

Converting to draft as we get more clarity from the OZ and Argent team on standards. @vixidev99 feel free to open this PR for review as we make progress on the specifications. Thanks!

[janek26]

moved to #87

@vixidev99 feedback appreciated !