[OSS] xDS updates for peerings control plane through mesh gw

[freddygv]

Description

Backport of ENT-3058

We want to enable peering control plane traffic through mesh gateways. This PR updates the xDS generation for OUTBOUND peering Establish connections. We want to create a route for each unique server cluster, which is identified by ServerName in the peering. We also want to make sure the operator has enabled PeerThroughMeshGateway in the Mesh Config Entry.

Testing & Reproduction steps

• Unit Tests added
• Manual Testing
  • Start two clusters
  • Register Mesh Gateway in the default namespace in cluster 1
  • Add Mesh CRD with PeerThroughMeshGateway in cluster 1
  • Generate a peering token in cluster 2
  • Establish from 1
  • Even though the gateway isn't receiving traffic, the xDS updates should be observed.

PR Checklist

• updated test coverage
• external facing docs updated
• not a security concern