Make key handling more ergonomic

[lrettig]

Here are some suggestions for how to improve the UX around key management:

1. Don't require --id to be passed on the command line if a key.bin file already exists. Read the key from the file.
2. Store key in the same format (probably hex, not base64) in key.bin and postdata_metadata.json (inconsistent representation of commitment ATX and node ID #163)
3. Make it easy to read the pubkey from key.bin (e.g., postcli -printKeys)

[fasmat]

I would go a different way about this by giving postcli a better CLI in general:

1. split the functionality it offers into multiple sub commands: postcli id (show and generate), postcli init, postcli verify, postcli genproof, postcli benchmark, postcli resize, etc.
2. Restructure the cmd package around those sub commands using cobra for a more structured and especially testable code: Introduce cobra for postcli #199
3. Don't infer anything that wasn't explicitly stated: e.g. if -id is specified but a key.bin file is present print an error and exit. if -id is not specified and no key.bin exist print an error and exit.

Changing the contents of postdata_metadata.json should be a separate issue, for multiple reasons:

• It's unrelated to CLI changes
• It requires existing data to be migrated (nodes currently store their nodeId in base64 if we change that to hex that will break existing PoST datas)
• postdata_metadata.json has no schema or similar (yet). So changing its contents in a non-backwards compatible way isn't trivial