Support multiple api urls and explorer urls per chain

We currently only allow a single `(api_url, explorer_url)` pair per chain.

Ideally we would per chain a way to define a type like

`Blockscout("foo", "bar")`,
`Etherscan("baz", "qux")`

To make it easier for users to select Blockscout (or others explorers) we should not require them to manually look up the respective URLs.