Spread the load between different RPC providers

[frol]

Currently, CLI uses a single rpc_url per network connection, but we often hit the problems if RPC provider is overloaded. We may consider several options:

1. Make rpc_url a list (API key needs to be also configured)
2. Create a fallback strategy, so when the selected network connection RPC is not working, it would try to send the request through another network connection which has the same network_name (mainnet / testnet / ...)

I believe the second option is better, and thus we will need to preconfigure more network connections by default.