Allow third party auth provider

[zboralski]

Example use :

var myProvider = &endpoints.AuthProviderConfig{
    IssuerID: "https://example.com",
    JWKSURI: "https://www.googleapis.com/service_accounts" +
        "/v1/metadata/raw/APPID@appspot.gserviceaccount.com",
    UserInfoEndpoint: "https://example.com/oauth2/v1/tokeninfo",
    ScopesSupported: []string{
        "openid",
        "email",
        "profile",
    },
}

func init() {
    endpoints.AuthProvider = myProvider
}