chore: Add signature to /solve response

[sunce86]

Background

As somewhat sensitive, will keep the issue description on slack: https://cowservices.slack.com/archives/C0375NV72SC/p1727269558467189

Bottom line, driver should use submission private key to sign arbitrary message (let's figure out what to sign, maybe each solution id?) and send the signature as part of the /solve response.

Driver will, then, be required to use the same submission key to execute submission if chosen as a winner.

Autopilot is supposed to extract the address from the signature and save it as a submission address to database. Later, circuit breaker will check if the settled solution is settled using the same address as extracted from the signature.

The alternative is using autopilot config to list the solver addressed #2780
I think we should use the signing as somewhat more future proof solution.

Details

All types of signatures should be supported. Meaning, driver should be allowed to settle solutions both with EOA and smart contract signer.

[sunce86]

For the purpose of proving that the solver is actually the owner of a submission address, it's irrelevant what is signed, as long as it's something deterministic and agreed on by both driver and autopilot.