Refactor: Move parsing into its own module

[carlaKC]

As is: we have a monolith main function that performs all parsing and creates a simulation.

Motivation: if using simln as a library, it would be nice to have a parsing module in sim-cli/src/parsing.rs to pull in some of the parsing/validation that main currently performs.

Some suggestions for things that we can pull out in a refactor:

• Cli: move into module so that it can be re-used externally
• SimParams: move to parsing module (it doesn't belong in the lib) (also move NodeConnection and ActivityParser)
• get_clients: builds the map of dyn LightningNode that'll be passed to the simulation
• validate_activities: performs ActivityParser -> ActivityDefinition conversion
• read_sim_path: this is a nice to have frill, might as well make it accessible for other folks

The only thing that's tricky here is that validate_activities needs to make calls to our dyn LightningNode to validate that pubkeys exist in their graph, so that needs to be threaded through. Ideally this could just be a generic closure that looks up the node, but it gets ugly when you start dealing with async closures (I briefly tried this in the past and gave up).