feat!: expose GoogleAuth constructor on AuthPlus class

[bcoe]

BREAKING CHANGE: pulls in breaking API changes in google-auth-library. getProjectId() and getProjectId() have been modified to make the impact of these changes less noticeable on the legacy googleapis module (getClient() is idempotent, but getProjectId() will use the last configuration).

At the end of the day, this allows us to avoid updating cloud.google.com and a a large number of auto generated samples that follow this format (not to mention stack overflow):

     * function authorize(callback) {
     *   google.auth.getClient({
     *     scopes: ['https://www.googleapis.com/auth/cloud-platform']
     *   }).then(client => {
     *     callback(client);
     *   }).catch(err => {
     *     console.error('authentication failed: ', err);
     *   });
     * }

I'm hoping this will give us the best of both worlds:

• where gRPC clients move towards the fully idempotent API (along with clients like PubSub).
• documentation in googleapis can be updated to teach the new API surface, however.
• we avoid breaking gobs of legacy code floating around a variety of tutorials.

[codecov]

Codecov Report

Merging #154 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #154   +/-   ##
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  Coverage   71.42%   71.42%           
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  Files           2        2           
  Lines         119      119           
  Branches       38       38           
=======================================
  Hits           85       85           
  Misses         16       16           
  Partials       18       18

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[JustinBeckwith]

I think I'm just more confused than anything about what's happening here

[JustinBeckwith]

Could I trouble you to go back into google-auth-library, and land a PR to export this properly?

[JustinBeckwith]

Isn't this exactly the kind of thing we want to avoid? Is this a method we would expect end users to consume?

[bcoe]

I don't want people to use this method, but I think it's a bit too sweeping to remove the method at this time. Here's why I think this implementation, for the benefit of googlepapis, is a bit better:

1. if I call getClient() multiple times, the prior call will not have an effect on the values returned by the second call (because a new GoogleAuth() is created in between).
2. if I call getClient(opts); getProjectId(); getClient(); getProjectId() the cached response from the first getClient won't have an effect on the second getProjectId().

[JustinBeckwith]

So this class extends GoogleAuth, and we're adding a reference to GoogleAuth itself? My head is spinnin a bit.

[bcoe]

I'm adding a reference to GoogleAuth, for the same reason that we added Oauth2, and Compute, so that we can advertise the approach:

const auth = new google.auth.GoogleAuth(opts);
const client = auth.getClient();

This allows me to advocate this methodology in the README as the safe approach to using the client going forward, also the API surface ends up being more consistent:

const oauth2Client = new google.auth.OAuth2(
  YOUR_CLIENT_ID,
  YOUR_CLIENT_SECRET,
  YOUR_REDIRECT_URL
);