It's not supported for the Url property. So I think you have to create a custom validator to block other content types, but that won't improve the user experience. It will only show an error message after the editor has tried to select invalid content types.

Don't we have any customized type for External links only? That can provide us full (Absolute) url.

Actually this is a need for meta tags.

Do you mean external links to CMS pages? It’s fairly straightforward to create absolute URL from page URL and add protocol, domain and, if needed, port. Just create UrlResolver or UrlHelper extension method that can be reused in all your projects. It’s better to leave that logic to the developer in my opinion.