Networking: Support for VPN clients

[neurolabs]

Motivation

Some development environments need access to private network resources via a VPN, often in corporate environments. Supporting all common VPN clients would strengthen adoption.

Proposed behavior

Next to existing support for OpenVPN/Tailscale (relying on existing TUN/VETH, add kernel support for VPN clients:

• Wireguard support: Enable kernel support (WIREGUARD, CRYPTO_CHACHA20POLY1305, CRYPTO_CURVE25519, CRYPTO_BLAKE2S, CRYPTO_POLY1305)
• (legacy) openfortivpn: Enable kernel support (PPP, PPP_ASYNC)
• (legacy) IPsec/strongswan: (XFRM_USER, INET_ESP, NET_KEY, INET6_ESP, CRYPTO_GCM)
• (legacy) L2TP: (L2TP)

The existing hook mechanism can be used to start the clients.

Alternatives considered

No response

[magro]

For openfortivpn I managed to adjust the Dockerfile so that the sandbox can use the host VPN connection (with appropriate networking config, with egress-allow set to hosts/CIDRs accessed via VPN.