Register OAuth 2 scheme in API index

[rmccue]

Adds .authentication.oauth2 to the index with URLs and detail about supported grant types:

{
    "authentication": {
        "oauth2": {
            "endpoints": {
                "authorization": "http://vagrant.local/wp/wp-login.php?action=oauth2_authorize",
                "token": "http://vagrant.local/wp-json/oauth2/token"
            },
            "grant_types": [
                "authorization_code",
                "implicit"
            ]
        }
    },
}