Decouple URLS and API keys in the prompt

**Is your feature request related to a problem? Please describe.**
Currently the Etherscan API key and the RPC URL of the nodes are deep inside the prompt. That makes it hard for people to provide their own RPCs and API keys if needed.

**Describe the solution you'd like**
Let's decouple them and add them at the end of the file. Also, maybe it makes sense to never post them on the assistant, but substitute them on the fly before running the commands.

Ideally the assistant should send something like `cast block --rpc-url YOUR_URL` and we would substitute it before running the command.