Google OAuth only provides refresh token on first access token

Problem/Motivation

We are using this module to connect on a long term basis to Google Drive. This means we need to periodically refresh access tokens. Google OAuth2 apparently only returns a refresh token when you exchange an authorization code for an access token; subsequent access tokens don't have the refresh token (see https://developers.google.com/identity/protocols/oauth2/web-server#offline). This means a second refresh fails because there's no refresh token on the first refreshed access token.

Steps to reproduce

1. Authorize the Drupal module (via /admin/config/system/oauth2-client) to access your Google Account. You receive an access token with refresh token.
2. Use the access token after expiration, triggering a refresh. The new access token does not have the refresh token in its data, and the AccessToken stored (in State in this case) has an empty refresh token.
3. Attempt to obtain a new access token after expiration. Refresh fails.

Proposed resolution

I can think of at least two possible resolutions:

1. Change Drupal\oauth2_client\Plugin\Oauth2GrantType\RefreshToken::getAccessToken() for all cases to check incoming AccessToken for a refresh token. If it's missing, set the refresh token on the new AccessToken to the existing one before saving.
   

           // Compensate for the fact that Google doesn't return a refresh token each time.
           if (empty($newAccessToken->getRefreshToken())) {
             $newAccessToken->setRefreshToken($refreshToken);
           }
   

2. Allow a hook_alter() on Oauth2GrantTypePluginManager so an implementing site can register its own RefreshToken grant plugin to do the above.

Remaining tasks

User interface changes

API changes

Data model changes