[google_sign_in] Bridge google_sign_in and googleapis.

[ditman]

Description

This new package extends the Google Sign In plugin with a Future<AuthClient> authenticatedClient() method, that lets you retrieve a googleapis_auth AuthClient from the currently authenticated googleSignIn instance:

...

import 'package:extension_google_sign_in_as_googleapis_auth/extension_google_sign_in_as_googleapis_auth.dart';

...

final peopleApi = PeopleApi(await _googleSignIn.authenticatedClient());
final response = await peopleApi.people.connections.list(
  'people/me',
  personFields: 'names',
);

...

This package also contains a copy of package:google_sign_in example app, modified to use the new method, and the googleapis package. This is what a refactor from raw HTTP requests to authenticated googleapis clients looks like.

Related Issues

n/a (Internal)

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]). This will ensure a smooth and quick review process.

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (flutter analyze) does not report any problems on my PR.
• I read and followed the Flutter Style Guide.
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy.
• I updated CHANGELOG.md to add a description of the change.
• I signed the CLA.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• No, this is not a breaking change.

[yjbanov]

RSLGTM