tools: add download_file_from_url

tools: add download_file_from_url

adds a common function to download arbitrary files from urls.

code is based on
manager:PVE/API2/Nodes.pm:aplinfo

Security notice: this function does not perform any permission
checking. The callee has to make sure, that only authorized users may
use this function.

Caution: This function is able to download files from internal
networks (which would not be visible/accessible from outside), the
callee needs to ensure that unprivileged (e.g., non root@pam or the
like) can only pass OK URLs (e.g., resolving to public routable IPs)

Signed-off-by: Lorenz Stechauner <l.stechauner@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>