Add support for shortcuts

[ajmas]

Google recently introduced the notion of shortcuts, with a file node resolving to a data type as follows:

{
  "id": "1TBGha4guewbMXMJrhYaH403hnuWT5LRO",
  "name": "My map",
  "mimeType": "application/vnd.google-apps.shortcut",
  "parents": [
    "1zLoGsLYASFMuEdkn0A0Kpr0xpCX21HcZ"
  ],
  "createdTime": "2020-07-02T18:03:18.769Z",
  "modifiedTime": "2020-07-02T18:03:18.769Z"
}

Support should be added for them

[ajmas]

Opened related issue: googleapis/google-api-nodejs-client#2258

[ajmas]

Right now there is provisional support in the project for shortcuts, but at this time I am not sure how to best create an integration test around it. The main issue is creating a resource that I can shortcut in the test folder.