Use a service account instead of OAuth

[ddonnini]

Hi Tobias!

I'd like to use a service account with this bundle in order to achieve a server-to-server situation.
https://developers.google.com/identity/protocols/OAuth2ServiceAccount

Right now it is achievable with a little trick. Use the bundle with a dummy config and then manipulate the GoogleClient.

$googleClient = $this->happyrGoogleClient->getGoogleClient();
$googleClient->setAuthConfigFile($pathToAuthConfigFile);

And everything works good. But I have all the required bundle config unused.

Maybe two new parameters (type and service_auth_config_file) and remove all requireness in other config fields?

What do you think?

[aschempp]

see #18