HG-225: Add caching headers for static assets

The `directory` \ `file` handler currently doesn't support changing headers (https://github.com/hapijs/inert/issues/11), This PR adds `Cache-control` and `X-Pass-Cache-Control` headers to the static assets. For the whole rationale behind setting the headers, please check the ticket comments :sparkler: 

do we have mechanism to purge cache for apis?