Ability to serve static files for paths different from `/`

[botanegg]

Add a description

For HttpHandlerStatic we cannot serve files for path different from root

Example, for next static_config:

    fs-cache-main:
      dir: /var/www          # Path to the directory with files
      update-period: 10s        # update cache each N seconds
      fs-task-processor: fs-task-processor  # Run it on blocking task processor

    handler-static: # Finally! Static handler.
      fs-cache-component: fs-cache-main
      path: /sub_path/d1/d2/d3/*                  # Registering handlers '/*' find files.
      method: GET              # Handle only GET requests.
      task_processor: main-task-processor  # Run it on CPU bound task processor

For request http://HOSTNAME/sub_path/d1/d2/d3/filename we will seach file /var/www/sub_path/d1/d2/d3/filename instead of intuitive /var/www/filename

It is due to HttpHandlerStatic using request.GetRequestPath() as path to search files in fs-cache:dir
To make possible to work on other base paths, we need to change file search path to GetPathArg