InteractionCallback's User object is missing `username`

[irgraham534]

https://github.com/slack-go/slack/blob/c4095cb17cf21c2b2b9c459d5512d14ca5143f64/interactions.go#L35-L71 https://github.com/slack-go/slack/blob/c4095cb17cf21c2b2b9c459d5512d14ca5143f64/users.go#L112-L139

Description

According to Slack's documentation for shortcuts and block_actions, the user object contained within the payload shown in the example should have username.

Regarding the shortcuts, specifically referring to the global shortcut example:

{
  "type": "shortcut",
  "token": "XXXXXXXXXXXXX",
  "action_ts": "1581106241.371594",
  "team": {
    "id": "TXXXXXXXX",
    "domain": "shortcuts-test"
  },
  "user": {
    "id": "UXXXXXXXXX",
    "username": "aman",
    "team_id": "TXXXXXXXX"
  },
  "callback_id": "shortcut_create_task",
  "trigger_id": "944799105734.773906753841.38b5894552bdd4a780554ee59d1f3638"
}