Implement BackendNotFound reason for backendRef that does not exist

[nathancoleman]

Will want to review + merge #290 first which this PR is based on

Changes proposed in this PR:

The contract for HTTPBackendRef has changed to include a new reason, BackendNotFound, for the ResolvedRefs condition when a known backendRef kind is used but the backend does not exist (docs). This change set implements that reason.

How I've tested this PR:

Created HTTPRoute w/ non-existent Service for backendRef, verifying that the new BackendNotFound reason is assigned to the status.

Example

By making a small change to the backendRef on this HTTPRoute from the Learn guide:

apiVersion: gateway.networking.k8s.io/v1alpha2
kind: HTTPRoute
metadata:
  name: example-route-2
  namespace: consul
spec:
  parentRefs:
  - name: api-gateway
  rules:
  - matches:
    - path:
        type: PathPrefix
        value: /
    backendRefs:
    - kind: Service
      name: nginx-not-here
      namespace: consul
      port: 80

### How I expect reviewers to test this PR:

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...)

[mikemorris]

LGTM to merge before #290, as it's not impacted by the potential upstream logic conflict described in #290 (review)

I think the usage of NewConsulResolutionError at

consul-api-gateway/internal/k8s/service/resolver.go

Line 257 in f19a977

return NewConsulResolutionError(fmt.Sprintf("consul service %s not found", namespacedName))

could be updated to instead return NewBackendNotFoundError too.

I was curious if there are any remaining valid uses of NewConsulResolutionError, or could if it be removed now, but I'm not sure if

consul-api-gateway/internal/k8s/service/resolver.go

Lines 280 to 283 in f19a977

	NewConsulResolutionError(fmt.Sprintf(
	"must have a single service map to a kubernetes service, found - (%q, %q) and (%q, %q)",
	serviceNamespace, serviceName, service.Namespace, service.Service,
	))

sufficiently maps to NewBackendNotFoundError to remove it entirely? Maybe?

Holding at least until #290 merges and this gets rebased for CI tests actually run.

Looks like there's a failing e2e test.

[mikemorris]

Force pushed because GitHub's auto-retarget didn't resolve the conflicts from squash-merging the prior base branch.

[andrewstucki]

@mikemorris yeah, I think that the line you commented here:

I was curious if there are any remaining valid uses of NewConsulResolutionError, or could if it be removed now, but I'm not sure if ... sufficiently maps to NewBackendNotFoundError to remove it entirely? Maybe?

is different enough from not being able to find a backend -- i.e. it found one, but problematically it found more than one -- that it probably should be kept its own error type.