Use the Open Fact Verifier mech to resolve markets

[evangriffiths]

See https://github.com/valory-xyz/mech/tree/main/packages/gnosis/customs/ofv_market_resolver

Some analysis done by the Gnosis AI team found the current resolver gets ~70% accuracy vs ~90% accuracy for this new mech. More detail in the PR description here valory-xyz/mech#225