feat(inspect): support policies for gateways

[michaelbeaumont]

Summary

This PR adds inspect API support for dataplanes that are builtin gateways.

The dataplanes/<dp>/policies endpoint now returns either:

{
  "kind": "SidecarDataplane",
  # what the endpoint returns on `master`
}

{
  "kind": "GatewayDataplane",
  # new result shape for gateway dataplanes
}

which is

kuma/pkg/api-server/types/gateway.go

Lines 32 to 36 in 3d17de3

	type GatewayDataplaneInspectResult struct {
	Kind string `json:"kind"`
	Listeners []GatewayListenerInspectEntry `json:"listeners"`
	Policies PolicyMap `json:"policies,omitempty"`
	}

The idea is to show for each GatewayRoute that applies to the Gateway served by this Dataplane, which policies apply to any destinations in that route. Applied TrafficLog and TrafficTrace policies are shown on the top level of the result.

An example might look like:

{
 "kind": "GatewayDataplane",
 "listeners": [
  {
   "port": 80,
   "protocol": "HTTP",
   "hosts": [
    {
     "hostName": "foo.com",
     "routes": [
      {
       "route": "default-foo-gateway-hw6n5",
       "destinations": [
        {
         "tags": {
          "kuma.io/service": "demo-app_kuma-demo_svc_5000"
         },
         "policies": {
          "CircuitBreaker": {
            ...
          },
          ...

Alternative

We don't necessarily have to return the gateway -> route tree.
Another option would to add return a list of attachments at the top level, like for sidecar dataplanes, with a new value for AttachmentEntry.type, which would encode the route information somehow.

Part of #3720

Changelog: feat(inspect): add gateways to policy inspect

[codecov-commenter]

Codecov Report

Merging #3916 (c9b2349) into master (beba90c) will increase coverage by 0.06%.
The diff coverage is 80.48%.

@@            Coverage Diff             @@
##           master    #3916      +/-   ##
==========================================
+ Coverage   55.82%   55.88%   +0.06%     
==========================================
  Files         917      918       +1     
  Lines       54748    54815      +67     
==========================================
+ Hits        30562    30634      +72     
+ Misses      21746    21734      -12     
- Partials     2440     2447       +7     

Impacted Files	Coverage Δ
pkg/plugins/runtime/gateway/route/table.go	100.00% <ø> (ø)
pkg/api-server/inspect_endpoints.go	72.04% <78.08%> (+3.13%)	⬆️
pkg/api-server/types/gateway.go	100.00% <100.00%> (ø)
pkg/api-server/types/inspect.go	76.92% <100.00%> (+0.45%)	⬆️
...plugins/runtime/gateway/gateway_route_generator.go	100.00% <100.00%> (ø)
pkg/core/tokens/default_signing_key.go	66.66% <0.00%> (-5.56%)	⬇️
pkg/core/resources/manager/cache.go	83.11% <0.00%> (-2.60%)	⬇️
pkg/insights/resyncer.go	73.61% <0.00%> (+2.45%)	⬆️
...s/authn/api-server/tokens/admin_token_bootstrap.go	82.00% <0.00%> (+4.00%)	⬆️
pkg/plugins/runtime/gateway/route/sorter.go	71.79% <0.00%> (+5.12%)	⬆️
... and 3 more

---

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