Switch to Blockscout

Asterisc currently [uses Etherscan](https://github.com/search?q=repo%3Aethereum-optimism%2Fasterisc%20etherscan&type=code) 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.