It fails with encrypted JWT tokens

[lmammino]

jwtinfo currently fails with encrypted tokens. It should instead print a nicer message mentioning that the token is encrypted and therefore it is not possible to read the body.

An example of this kind of token is what's generated by AWS Cognito as a refresh token.

The header part looks like the following:

{
  "cty": "JWT",
  "enc": "A256GCM",
  "alg": "RSA-OAEP"
}

On possible solution is that the CLI should not try to parse the body as JSON if there is an enc field in the header.