Return list of supported APIs

[ryanseys]

Maybe something like:

var google = require('googleapis');
google.getSupportedAPIs(); // { drive: ["v1", "v2"], adsense: ["v1beta"] } 

[h2non]

+1 interface discovery is always useful.

Maybe also something like:

google.hasAPISupport('v1')
// or
google.isSupported('v1')

[grant]

How about just exporting this object? @jmdobry
https://github.com/google/google-api-nodejs-client/blob/master/src/apis/index.ts#L222

export const APIS: APIList = {
  abusiveexperiencereport: {
    'v1': abusiveexperiencereport_v1.Abusiveexperiencereport,
  },
  acceleratedmobilepageurl: {
    'v1': acceleratedmobilepageurl_v1.Acceleratedmobilepageurl,
  },
  adexchangebuyer: {
    'v1.2': adexchangebuyer_v1_2.Adexchangebuyer,
    'v1.3': adexchangebuyer_v1_3.Adexchangebuyer,
    'v1.4': adexchangebuyer_v1_4.Adexchangebuyer,
  },
  // ...
}