zone aware traffic

[szuecs]

We want to build zone aware routing into skipper, routesrv to reduce costs and latency.

related issues:

• endpointslices support zone aware routing #2552
• Support endpointslices - Prefer Same zone traffic to save costs #1446

dataclient/kubernetes add zone data: #3238

related PRs to show case different options:

• option 1 feature: zone aware routing - Option 1 #3117
• option 2 dataclients/kubernetes: add metadata route - zone aware option 2 #2845
• option 3 feature: push endpoints zone aware routing option3 #3233
• option 4 feature: endpointregistry fetches from zone-aware routesrv API - option 4 #3258

[szuecs]

Everything is implemented also in routesrv so also big setups can do zone aware traffic.