Add runtime support for Ethereum network configuration

[kdeme]

Support for Portal testnets got added here: #2206

However, once different data than the mainnet data gets used on a Portal network, the node will also need to be able to select which network config to use per network.

[bhartnett]

I guess this config selection should be based on the selected --network so selecting a network that contains non-mainnet data would trigger reading in the matching config.

One thing to think about is how should Fluffy handle local networks when selecting --network=none. Currently we default to using mainnet config but what if you want to test different data in a private network.

[kdeme]

One thing to think about is how should Fluffy handle local networks when selecting --network=none. Currently we default to using mainnet config but what if you want to test different data in a private network.

Yes, we don't have thought this through currently. In the case of network none we need extra way to say which config.

Perhaps we add an additional --network-config flag specifically for this setup.
Or additional info can get added in the network flag, e.g. --network=none:mainnet or something like that. We could also rename none to custom or similar. Open to ideas here...