DNS Prefetching + Preconnect of assets_host (CDN Domain)

What does this MR do?

This adds as a <link rel="dns-prefetch" href="//assets.gitlab.com"> dns-prefetch hint the setup asset_host so the browser will try to prefetch already the DNS for the CDN domain. It also adds a preconnect hint so that the connection to the CDN is made as fast as possible <link href="//assets.gitlab.com" rel="preconnect" crossorigin>

More information:

https://www.keycdn.com/blog/resource-hints/

https://w3c.github.io/resource-hints/

Why was this MR needed?

To improve performance especially on first load

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Conforms to the code review guidelines
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #49107 (closed)