Add ability to load token_json from string

[tomcodes]

Hi!

I have been using this package successfully to manage Google Calendar sync. I currently have several hundreds users relying on it and have started migrating the users to a more simple OAuth setup.

Currently, this package provides the ability to load an access token by file only (google-calendar.auth_profiles.oauth.token_json).

Since I store the OAuth tokens in DB I would like to be able to provide the tokens by string. Is it something you might be interested in? I could try and start working on a PR.

Thanks!

[antoinelame]

Interested!

I believed it has been done though, but not merged: #232