4.x: backport static content changes to Reactive

[tomas-langer]

Once PR #5543 is merged, we should update implementation of Helidon Reactive as well.
It would be best if we could have a single shared library used both from Níma and Reactive, but that is limited by the request and response APIs of WebServers.

At least the builder API should be similar, and we should be able to reuse the caching mechanism (I hope).

This should be then backported to 3.x and 2.x (e.g. we should keep backward compatibility of the API)

[barchetta]

No longer applicable.