make sure gashawk can never access private key

[malteish]

Currently, the signer object is handed to Gashawk:

const signer = new Wallet(PRIVATE_KEY, ethers.getDefaultProvider());
const gashawk = await Gashawk.fromSigner(signer, API_URL);
const gashawkSigner = gashawk.getSigner();

Since the signer object contains the user's private key, this could cause trust issues. In order to avoid these, implement signing the terms of service like this:

1. request terms or service message from Gashawk
2. sign message with signer
3. return signature to Gashawk
   steps 2 and 3 can probably be done in 1 line of code, so there is only one extra line of code as cost for resolving the trust issue.

Update: I'm not sure I understood the architecture good enough. We need to talk about it.