feat: hide handling cache headers behind the faeture flag

What does this MR do?

We recently changed how we serve files in !529 (merged), now we support handling cache headers, even though we don't set them anywhere yet.

vfsServing.ServeCompressedFile logic is quite complex, and I'm afraid that we may handle something not the right way.

So I want to hide this behind the feature flag before cutting the release.

This is based on !642 (merged)

TODO

• I added the Changelog trailer (e.g. Changelog: feature) to the commits that need to be included in the changelog
• I added unit tests or they are not required
• I added acceptance tests or they are not required
• I added documentation (or it's not required)
• I followed code review guidelines
• I followed Go Style guidelines