xDS Example

[markmandel]

It would be lovely to have some kind of simple xDS example in https://github.com/googleforgames/quilkin/tree/main/examples

One potential idea I had, was to do something on the client side proxy, that adds the hosted endpoint to the client proxy. I thought this might be a simple enough example. Just a thought though.

[markmandel]

I'm lumping into this ticket the work converting xDS into Quillkin itself - since it solves both problems at once.

[markmandel]

I have this locally with Agones + Quilkin + Xonotic. Just waiting on #574 to review and see if simplifies any of the configuration.