Support Google introspection endpoint (#1590)

* Support Google introspection endpoint

Google doesn't follow the usual RFC 7662 format but instead uses their
own. This PR adds support for parsgin their format if using a well-known
Google endpoint.

To test:
 - Go to Google OAuth 2.0 Playground
 - Select the scopes you want (e.g. `https://www.googleapis.com/auth/userinfo.email`)
 - Click "Authorize APIs" and complete the OAuth flow

You'll get an access token that you can copy and use in the inspector.

Fixes: #1411

* Add providers

* Use LimitReader to avoid DoS by evil endpoints

Author: jhrozek