Add username to atc output

[CharlieAtSchneider]

This PR adds username to jf atc output. Since it isn't appearing in the reply to the API call, it is instead manually appended to the output in code.

Please tell me if you think the idea is sound. If so, I can fix my issue in the Caveats section below.

Examples:
jf atc:

{
  "access_token": "<TOKEN>",
  "expires_in": 15552000,
  "scope": "applied-permissions/user",
  "token_id": "<ID>",
  "token_type": "Bearer",
  "username": "<MY_USERNAME>"
}

jf atc --format=table

FIELD         VALUE
access_token  <TOKEN>
token_id      <ID>
expires_in    1.5552e+07
scope         applied-permissions/user
token_type    Bearer
username      <MY_USERNAME>

Use case

If you can't run jf docker commands, (in our case due to VM shenanigans) you may need to have a username together with the token to log in to Docker. This addition to jf atc would fix this.

Caveats:

• I am not a Go developer, and I created this code by looking in the source code for reference. Please correct me if I made any simple mistakes.
• I'm not super fond of the idea of running json.Unmarshal(), add a username field to the struct, turn it back with json.Marshal() for it to just turn back with json.Unmarshal() later in the code in other functions that I didn't touch. What do you think - should this be avoided with the help of some refactoring?

Tests, formatting etc

• All tests have passed. If this feature is not already covered by the tests, new tests have been added.
• The pull request is targeting the master branch.
• The code has been validated to compile successfully by running go vet ./....
• The code has been formatted properly using go fmt ./....

All code written by a human

[github-actions]

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[CharlieAtSchneider]

I have read the CLA Document and I hereby sign the CLA