This patch adds a get secret subcommand. With no args, lists holos
secrets. With args, gets each argument.
The use cases are:
1. Extract specified keys to files with --to-file
2. Extract all keys to files with --extract-all
3. Print one key to stdout with --print-key
If no key is specified, the key is implicitly set to the holos secret
name. This behavior should be preserved as part of the api.
