Allow dumping both header and body

[bsmedberg-xometry]

I'd like to add a feature to jwtinfo that allows dumping both the header and the body simultaneously, perhaps a --full flag? @lmammino would you accept a PR for this?

Output:

{
  "header": {
    "kid": "24378254378"
  },
  "claims": {
    "iat": 12345,
    ...,
  }
}

[lmammino]

I have been thinking about something like this for a while and I think it would be a very convenient addition to the project, so I'd be super happy to receive a PR if you have the time to contribute :)