Support multichain workflows in Cast

### Component

Cast

### Describe the feature you would like

Cast is a great utility for a variety of tasks, but it's only geared towards workflows that use a single RPC endpoint. For multichain workflows, you talk to multiple chains, and often you are trying to cross-reference information across chains or perform actions on multiple chains at once.

My proposal is to extend Cast with multichain support. My idea here is that anything that requires `--rpc-url` can now take multiple instances of that option. The action call is then made on each RPC.

This also enables cross-referencing information between nodes that aren't necessarily on different chains, e.g. for differential testing of RPC methods across client implementations.

For the environment variable, I think `clap` supports comma-delimited values for options that are `Vec<_>`.

### Additional context

Some Cast commands I'd particularly like to see this on:

- `cast send`
- `cast call`
- `cast code`
- `cast nonce`
- `cast block`

Start in the Cast CLI entry points for --rpc-url and trace how the option reaches cast send, call, code, nonce, and block. Check how clap handles Vec values and how the environment variable is parsed; done means these commands can accept multiple RPC endpoints and perform the requested action for each.

Written by the indexing model from the issue text.