nodebuilder/node: Init should also generate and save node ID and JWT secret

[renaynay]

Init initialises the node directory, but is still missing some components:

• Node ID
• JWT secret

Users should be able to run celestia <node_type> init and for all keys to be present such that they can use their node before it's running on a network.

cc @cmwaters

[Wondertan]

Or we should remove init altogther in favor of lazy init from start

[renaynay]

I would prefer not to remove it before doing a bit of research with node operators to see how many actually prefer the initialisation step to be separate.