Adds implementation fetch

[mcgingras]

Great service so far! I really like it. However, I noticed that when I tried to fetch the ABI for contracts that are proxies, it does not return the ABI of the implementation. The ABI of the implementation is useful since it describes the proxy as well. I've added a bit of code that will fetch the ABI of the implementation (if it exists) and return it.

This PR is a spike -- more to start conversation around this feature set and show a minimal POC. If this is something you are interested in I think we will need to update the viem publicClients so it supports all of the networks you are currently supporting. It shouldn't be too bad, but I didn't want to do it unless you think this is something that you'd want to add into your service. Happy to take it on if this seems interesting to you though!

Opening this PR to get the convo going, if it's not something you are interested in, no problem, I'll just host this myself.

Example proxy contract if you want to test (on mainnet): 0x1eD51f1c554D132B5f62bf7b510eA4bD3e7f004E