feat: add contender admin contract-address subcommand

[Peponks9]

closes #411

open for feedback :)

cc @zeroXbrock

[zeroXbrock]

thanks @Peponks9, nice work!
I just made a minor adjustment: I realized we were using String for urls in a lot of places where we should have been using url::Url, so I went ahead and refactored all the urls. Serde can parse Urls natively so we don't have to worry about parsing the strings ourselves, and storing urls this way prevents slightly-differently-formatted rpc URLs (like a trailing slash) from screwing up the admin contract-address request.