feat(kuma-cp) provide total field when listing resources in the HTTP API

[nickolaev]

Summary

When we fetch entities via the HTTP API (and therefore via the GUI and kumactl) we don't know what is the total number of sites for a particular resource stored in the Kuma control plane. Return a total count of items for each resource that works on both Universal and Kubernetes.

Adds a new total top-level property in the HTTP API responses, so they look like

$ curl localhost:5681/meshes
{
 "total": 3,
 "items": [
  {
   "type": "Mesh",
   "name": "default",
   ...
  },
  {
   "type": "Mesh",
   "name": "default1",
   ...
  },
  {
   "type": "Mesh",
   "name": "default2",
   ...
  }
 ],
 "next": null
}

The implementation in all supported resource store mechanisms (memory, PostgreSQL and Kubernetes) is based on naive resource counting each time a List requests gets in. That can be optimized (cached?) for better scalability once Kuma has to handle huge amount of resources with multiple concurrent API requests happening. This optimisation is left for a follow-up PR.

Full changelog

• feat(kuma-cp) add naive total implementation for k8s store
• fix(kuma-cp) 32 bits for Total should be enough
• chore(*) reorder to make Total on the top of the structure
• feat(kuma-cp) add support for total count in PostgreSQL store
• (http/feat/count_entities) fix(*) fix testing when totals introduced
• feat(kuma-cp) expand ResourceList interface

Issues resolved

None

[bloqhead]

I've got this working in the GUI. It can be tested in this PR.

[CLAassistant]

All committers have signed the CLA.

[jakubdyszkiewicz]

Can you check if total elements counting works with different meshes?
Create 2 meshes: "default" and "demo"
Create 2 traffic permissions in "default" mesh and in 2 in demo
List traffic permissions by "demo" and see if the total is 2 or 4.

[nickolaev]

Right, so I need additional filtering when counting the resources here.

[jakubdyszkiewicz]

Please add a test in store_test_template.go it's a test that checks the contract for all the stores, so you will write the test once for all stores.