Interal URLs cannot be used as oauth provider

The default instance of `UrlValidator` prevents us to use intranet domain names, which are not compliant to url validator. For example dev.somewhere.local is invalid because of local is not valid TLD.

```groovy
class SpringSecurityOAuth2Controller {
...
def authenticate() {
String providerName = params.provider
if (StringUtils.isBlank(providerName)) {
throw new OAuth2Exception("No provider defined")
}
log.debug "authenticate ${providerName}"
String url = springSecurityOauth2BaseService.getAuthorizationUrl(providerName)
log.debug "redirect url from s2oauthservice=${url}"

//You cannot use internal/local urls because of this line
if (!UrlValidator.instance.isValid(url)) {
flash.message = "Authorization url for provider '${providerName}' is invalid."
redirect(controller: 'login', action: 'index')
}
redirect(url: url)
}
...
```
It is possible/reasonable to omit the validation or make it configurable e.g. by injecting the validator?

Start in SpringSecurityOAuth2Controller.authenticate, focusing on the UrlValidator.instance check shown in the issue. Review how the OAuth2 authorization URL is obtained, then clarify whether validation should be omitted or made configurable; done means the agreed behavior supports the stated internal URL use without rejecting valid provider URLs.

Written by the indexing model from the issue text.