MeshExternalService should aggregate different services into one

[lukidzi]

Description

Let's say we have Redis instances in zone-a (K8s), zone-b (K8s), and zone-c (universal). If we want to aggregate all these Redis instances from different zones, we can't achieve this with the current MeshExternalService. Once added on global, specific MeshExternalService will work on k8s but not universal. Allowing users to aggregate many different MeshExternalServices into one would be nice.

Use case:

• MeshLoadbalancingStrategy and fallback to a specific zone

type: MeshExternalService
name: redis
mesh: default
spec:
  match:
    type: HostnameGenerator
    port: 6379
    protocol: tcp
  endpoints:
    - address: redis.db.svc.cluster.local

Ideas:

• MeshMultiZoneExternalService?

[jakubdyszkiewicz]

Triage: It's about fallback of multiple MeshExternalServices. A couple of ideas

1. Improve MeshLoadbalancingStrategy to accept multiple different MeshExternalServices
2. Aggregate multiple MeshExternalServices into MeshMultiZoneService using meshExternalService selectors
3. MeshMultiZoneExternalService