[Enhancement] Squid proxy for restricted environments

[vendito-david]

Certain restricted environments have no internet connectivity and updating the image is problematic in those environments with rsync.

The OpenVAS image could use the existing SSH tunnel (forward a local port next to the unix socket) to access a Squid proxy running on the main GVM instance. Starting the proxy would be optional and in case it is not started, it wouldn't be possible to access it from the OpenVAS.

Standalone proxy could be also utilized with extra work, but this way only a single SSH connection is required for those restricted environments.

[vendito-david]

Paired with issue #344 in GVM-Docker.
Secure-Compliance-Solutions-LLC/GVM-Docker#344