cmd: A flag to set custom private key for p2p communications

[Wondertan]

Right now, the p2p private key is generated and saved on the first run, but it should be possible to set it from the CLI flags to facilitate testing utilities and Node deployment by excluding indeterministic key generation. Instead, the infrastructure should control non-determinism and be able to pre-generate the key using cel-shed and apply it to the Node.

[renaynay]

Flag should direct user to pass a path to key file.