add internal proxy for private files when no other option is available

[ccancellieri]

Hi as discussed this can be used as fallback when no other access is provided and untill better support for other drivers will be provided. This allo to access private files and serve content synchronously (!). It's not platform dependent. It adds an option (by default false) to enable proxy.

I will provide separately google cloud temporary encrypted url functionality.

Related to #49 and #51