Make DNP_NOTIFICATIONS & DNP_PREMIUM available while installing dappnode

[Marketen]

During DAppNode installation, all core packages (manifests, docker-compose files, and images) are downloaded from GitHub.

https://github.com/dappnode/DAppNode/blob/marc/ports80-443-https-running/scripts/dappnode_install.sh#L140

This approach fails when a core package is hosted in a private (non–open source) repository, since the installer cannot access it. This currently affects the new DNP_NOTIFICATIONS and DNP_PREMIUM packages.

A possible solution is to provide a public URL that hosts the required installation artifacts (manifest, compose files, images) for DNP_NOTIFICATIONS and DNP_PREMIUM, allowing the install script to download them during setup.