Support custom networks in sncast verify

[cptartur]

Current State

Right now, sncast verify only supports predefined network. However, some providers like walnut supposedly support verification for custom networks as well.

Objective

Add support for custom networks in sncast verify

Additional Context

How custom networks support works within the verification api will have to be verified.

[EjembiEmmanuel]

Can I be assigned to this? I'm a Cairo and Rust developer with experience using sncast to interact with the blockchain. I'd tackle this issue by going through the contract verification feature on the sncast library to understand the best to implement support for custom networks.