Add Targets field to service resolver failovers.

[erichaberkorn]

Description

The Targets field is more expressive than the previous way failovers were configured because it allows users to express complicated failover scenarios such as:

Kind           = "service-resolver"
Name           = "web"
ConnectTimeout = "15s"
Failover = {
  "*" = {
    Targets = [
      {
        PeerName = "cluster-01"
        Service = "web-from-peer"
        Namespace = "peer-namespace"
      },
      {
        Datacenter = "dc2"
      }
    ]
  }
}

It will be used to add failover support to partitions and cluster peers.

Testing & Reproduction steps

• Unit tests - For now this just changes the structs and isn't used anywhere.

PR Checklist

• updated test coverage
• external facing docs updated - I'll update the documentation when this feature is complete.
• not a security concern

[dhiaayachi]

LGTM!