Backport of [API Gateway] Fix use of virtual resolvers in HTTPRoutes into release/1.15.x

[hc-github-team-consul-core]

Backport

This PR is auto-generated from #17055 to be assessed for backporting due to the inclusion of the label backport/1.15.

The below text is copied from the body of the original PR.

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Description

I realized that we were actually improperly munging http route references in synthesized discovery chains for API gateways. What this meant was that when using an service resolver to create a virtual service referenced by an HTTPRoute we wind up writing in the wrong target cluster for a routing rule. This results in the gateway returning "no healthy upstreams".

This change takes into account when the target is just a virtual service created by a service-resolver.

Testing & Reproduction steps

Unit tested and manually verified with the following configuration entries and a mesh-federated Consul setup:

1. TCP Routes

Kind = "tcp-route"
Name = "api-gateway-route"
Services = [
  {
    Name = "tcp-dc-2"
  }
]

Parents = [
  {
    Name = "api"
  }
]

Kind = "api-gateway"
Name = "api"
Listeners = [
  {
    Name     = "listener-one"
    Port     = ...
    Protocol = "tcp"
  }
]

Kind = "service-resolver"
Name = "tcp-dc-2"
Redirect {
  Service    = "tcp-2"
  Datacenter = "dc2"
}

2. HTTPRoutes

Kind = "http-route"
Name = "api-gateway-route"
Services = [
  {
    Name = "http-dc-2"
  }
]

Parents = [
  {
    Name = "api"
  }
]

Kind = "api-gateway"
Name = "api"
Listeners = [
  {
    Name     = "listener-one"
    Port     = ...
    Protocol = "http"
  }
]

Kind = "service-resolver"
Name = "http-dc-2"
Redirect {
  Service    = "http-2"
  Datacenter = "dc2"
}

PR Checklist

• updated test coverage
• external facing docs updated
• appropriate backport labels added
• not a security concern

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Overview of commits

• ee9fd90 - 6bc25b5

[github-team-consul-core-pr-approver]

Auto approved Consul Bot automated PR