Add download/upload conf support

[DrmnSamoLiu]

When I was looking at uhttpd and dsd, I realized there are two URLs : "/admin/system/download_conf" and ""/admin/system/download_conf"". Also there are two functions named download_conf and upload_conf in uhttpd, with some obvious DES encryption.

Unfortunately I'm super bad at tracing and reconstructing code for now so I have no idea how this API should look like. So I'm hoping the great reverse engineers here can take a look and see if it's possible to implement in pytapo :)