[Tech Debt] Allow nodes to rejoin by saving index and config

[rob-maron]

Closes #2135

This PR:

Allows nodes to rejoin consensus after stopping and dropping off completely.

• Adds from_file and to_file to the NetworkConfig implementation
• Adds from_file_or_orchestrator to the above implementation, which takes an orchestrator client and returns both the configuration and the source (so we don't make the Ready call to the orchestrator every time)
• Cleans up a lot of OrchestratorClient code

There are also some unrelated changes that make running the examples easier.

This PR does not:

• Edit any example functionality
• Add a default config file for examples that run the orchestrator, webserver, and nodes (all_xxxx)

Key places to review:

• config.rs - the NetworkConfig implementations
• examples/mod.rs - for how we use the new NetworkConfig API in our examples

[dailinsubjam]

LGTM!

[shenkeyao]

LGTM, just a few minor comments.

[shenkeyao]

LGTM!