Enhance ExperienceFragments and ExperienceFragmentDataImpl to support microsites

## Feature Request

**Is your feature request related to a problem? Please describe.**
Is not possible to use Experience Fragments (e.g. for header/footer) for microsites or website that don't use standard ISO codes.

**Describe the solution you'd like**
It would be nice to have ExperienceFragmentData interface available and not only Implementation class in order to override some specific methods (for example: getLocalizedFragmentVariationPath() to support paths like /content/myproject/global/microsite) with delegation pattern.

**Are there alternatives?**
The alternative for me for now is simply to copy paste the core component code under our custom repository and simply modify what we need.

Locate ExperienceFragments and ExperienceFragmentDataImpl and read how getLocalizedFragmentVariationPath() is currently provided. Define the interface and delegation-based extension point described in the request, then verify that microsite paths such as /content/myproject/global/microsite can be supported without copying the core component code.

Written by the indexing model from the issue text.