Support non-mainnet chains

[kevinhalliday]

There are two approaches we could take. I suggest we take them in sequence.

1. Right now we support just one provider url. We assume that url to represent mainnet. With excercise that assumption a few places in code, but not many. Instead, we should allow users to provider any rpc url, and infer a single, constant chain id from that. This would be quick to implement.

2. The above approach would still enforce users can only use a single chain at a time. So switching chains while using a dapp would continue not to work. We could support multiple chains with the following changes:

   • allow input of chainId in fork creation config
   • allow input of provider url on fork creation. maintain some mapping of chainId to default provider rpc for convenience
   • update active fork when chainId changes