Feature: Add ability to run with mocked lightning network

[carlaKC]

Depends on #214

As is: We have a fully "mocked" implementation of LightningNode in sim_node that just keeps track of channel state in memory to create very lightweight simulations, but it is not currently surfaced in sim-cli so there is no way for people to easily use it.

Motivation: This is a great feature to have for larger networks, so we should surface a way for people to use it.

To Be:
We need to update our sim params to add a new sim_network that specifies the channel policies of the network that we want simulated. This option is mutually exclusive with nodes, because we can't run real and fake nodes together.

I have a mega-branch that took a shot at this but I don't like the approach I took (and the branch is a mess). This commit is useful because it provides the json structure that I think that we should use.

Option to run sim-ln with then be:

• nodes + activity: run defined activity on real nodes
• nodes: run random activity on real nodes
• sim_network + activity: run defined activity on fake nodes
• sim_network: run random activity on fake nodes

(all else is invalid)