Allow retrieving token via credentials

[timkurvers]

Although not a preferred OAuth or OpenID flow, the resource owner credentials grant allows retrieving a token via credentials without user interaction (e.g. opening up a browser window with the identity provider's log in page).

As such it can be used to validate credentials with the identity provider:

token = OpenIDTokenProxy.client.retrieve_token!(
  username: 'foo',
  password: 'bar'
)
token.validate!