mesh-vpn: fully abstract VPN methods

This fully abstracts VPN methods, making gluon-mesh-vpn-fastd and
gluon-mesh-vpn-tunneldigger completely self-contained.

Provide a LUA interface for generic interacting with VPN methods in
gluon-mesh-vpn-core and web packages.

This also adds the ability to install tunneldigger and fastd to the same
image, selecting the VPN method based on the selected domain.

Signed-off-by: David Bauer <mail@david-bauer.net>