Switch to Blockscout

[fainashalts]

Asterisc currently uses Etherscan for contract verification and we need to switch it to Blockscout. We have whitelabeled our explorer URLs so that a change to the explorer in the future won't require changes in the code.

This needs fairly minimal changes to the verification script:

--verifier blockscout 
--verifier-url { https://explorer.optimism.io/api/ | https://testnet-explorer.optimism.io/api }

I don't believe you need an API key for blockscout verification.