Give example of key generation with saving keys

I'd like to see something like this, couldn't figure out how to save generated keys in GPG or KBX formats...

```ruby
# generate both a primary key and a subkey
generated = rnp.generate_key(
  primary: {
    type: 'RSA,
    length: 2048,
    userid: 'testuser',
    usage: [:sign]
  },
  sub: {
    type: 'RSA,
    length: 2048,
    usage: [:encrypt]
  }
)

# save??
```