add user debug mode

User can send `X-Zerovm-Debug: true` header in request to request debug data
The reponse to request will consist of a responses from all backends concatenated into one tar file
All the data that backends send to the proxy middleware will be present in the response
You will need a proper tar stream/file reader to produce separate files from the response body
Response body MAY have files with the same name from different hosts
Each host data is delimitered by tar 'null section', which is 2 or more 512 byte blocks of zeroes.