Make it possible for FrontendServlet to process returned resources

[steinarb]

The FrontendServlet currently returns the resources it finds as is.

When the returned file is the index.html that bootstraps my webapp, I need to set some opengraph tags based on the entry point of the application.

I.e. be able to give the URL to a specfic react router route, and have the HTML of that route return appropriate opengraph tags.

It would be nice if the FrontendServlet could support this use-case.