Set defaults for private key generation

Private key generation via cli is quite verbose:

turnkey pk create --key-name some-key --address-format ADDRESS_FORMAT_ETHEREUM --curve CURVE_SECP256K1 --name some-name

We should consider making --address-format and --curve optional, with defaults set to ADDRESS_FORMAT_ETHEREUM and CURVE_SECP256K1 respectively.

Furthermore, we could probably flesh out the flags:

private-keys interacts with private keys

Usage:
  turnkey private-keys [command]

Aliases:
  private-keys, pk

Available Commands:
  create      create a new private key
  list        list private keys for the organization

Flags:
  -h, --help   help for private-keys

Global Flags:
  -k, --key-name string       name of API key with which to interact with the Turnkey API service (default "default")
  -d, --keys-folder string    directory in which to locate keys (default "/Users/andrew/Library/Application Support/turnkey/keys")
      --organization string   organization ID to be used
  -o, --output string         output format (default "json")

Use "turnkey private-keys [command] --help" for more information about a command.

Start with the CLI entry points for turnkey private-keys create and its pk alias, then inspect the existing flags and help output. Done means --address-format and --curve are optional with the requested defaults, and the private-key command help clearly documents the available commands and flags.