add support for VHDS

[roivaz]

VirtualHost service discovery is available in envoy and go-control-plane already has support for it. Add, if possible, support for it in Marin3r.

/kind feature
/priority important-longterm
/assign

[roivaz]

I have investigated a bit about this and there are several points to consider:

• VHDS is only available when using the delta xDS. We currently the sotw (state of the world) variant of the aggregated xDS protocol, mainly because it ensures certain assurances about the correct ordering of resources when they are sent to the client (see https://www.envoyproxy.io/docs/envoy/latest/configuration/overview/xds_api#aggregated-discovery-service). I have done a quick test and have checked that the delta variant of the ads protocol works properly in Marin3r. This would require thorough testing though, as there are several open issues in go-control-plane regarding the delta xDS (https://github.com/envoyproxy/go-control-plane/issues?q=is%3Aissue+is%3Aopen+delta)
• The big blocker to implement this is that currently VHDS does not support the aggregated variant, see VHDS Support envoyproxy/go-control-plane#525 (comment). We could register a non-aggregated delta VHDS discovery service with the server, but I'm unsure how that would play in conjuction with all the other resource types being feed by an aggregated xDS ...