Investigate non-public dataspace deployments

In certain scenarios, companies deploying the connector might not want to have the connector accessible via the internet. We should investigate whether this can be done and whether this would not eliminate all benefits of a dataspace.

Current ideas:

• Tailscale / Headscale (where network traffic is directly between participants) setup, or a trusted VPN (where network traffic is routed via the VPN server) setup, to create a private network between participants
• Message broker at a trusted third party with encrypted messages. Also, a DID method should be chosen in this case that doesn't require resolving the DID document at the private instances. Should only be done if strictly necessary due to project obligations.