Support routing to Consul services imported from a peer

[nathancoleman]

Changes proposed in this PR:

• Add peer to the MeshService CRD enabling users to target a Consul service imported from a peering connection
• Use peering endpoint instead of local catalog to resolve route backends that specify a peer

How I've tested this PR:

• Follow Cluster Peering on Kubernetes guide
• Install services into one datacenter and export one or all of them to the other datacenter (the local track of this guide is handy)
• In the other datacenter (the one not containing the services), create a Gateway and HTTPRoute with a MeshService backend referencing one of the services imported from the other datacenter
• Verify that you can reach the imported service through the Gateway

How I expect reviewers to test this PR:

See above

Checklist:

• [ ] Tests added
• [ ] CHANGELOG entry added

  Run make changelog-entry for guidance in authoring a changelog entry, and commit the resulting file, which should have a name matching your PR number. Entries should use imperative present tense (e.g. Add support for...)