feat(kuma-cp) add GlobalInsights api endpoint

[bartsmykla]

Summary

/global-insights is an endpoint which will return aggregated
statistics for global resources, like Zone, ZoneIngress or
Mesh. It's purpose is simmilar to the MeshInsights, but for
global resources, and will be used in our GUI to simplify
displaying statistics of these objects.

I've spent to much time thinking about the approach for adding
this endpoint and finally decided to go with the easiest one,
which is just asking for Zones, ZoneIngresses and Meshes
every time endpoint is hitted as the GlobalInsights is different
than other resources like MeshInsights as there always will be
one. The assumption is also, that there won't be a lot of
the resources we'll be asking for every endpoint hit.

Other solutions considered were:

1. Adding GlobalInsights as the real resource, like
   MeshInsights, but then we would have to create separate k8s'
   CRD, for just the resource which would exist only one at a time.
   Also, as it's the global resource, reconciliation of it would
   be more complex.
2. Storing the internal data in Config resource (which maps to
   json string in k8s' ConfigMap), but I really think it wouldn't
   be right, as I think it would be not obvious and harder to
   maintain in the future. The complexity of reconciliation would
   be the same as in the option above.

I think this solution is very easy, and if anyhow it will become
performance bottleneck, we can just improve it/refactor it, without
breaking backward compatibility.

Full changelog

no changelog

Issues resolved

no issue resolved

Documentation

• Link to the website: docs(api) add Global Insights endpoint to api ref kuma-website#572

Testing

• Unit tests
• E2E tests
• Manual testing on Universal
• Manual testing on Kubernetes

Backwards compatibility

• Add backport-to-stable label if the code is backwards compatible. Otherwise, list breaking changes.

[codecov-commenter]

Codecov Report

Merging #3018 (d58a417) into master (9aeea34) will increase coverage by 0.03%.
The diff coverage is 68.57%.

@@            Coverage Diff             @@
##           master    #3018      +/-   ##
==========================================
+ Coverage   52.25%   52.29%   +0.03%     
==========================================
  Files         913      914       +1     
  Lines       52906    52941      +35     
==========================================
+ Hits        27645    27684      +39     
+ Misses      23059    23050       -9     
- Partials     2202     2207       +5     

Impacted Files	Coverage Δ
pkg/api-server/global_insights_endpoints.go	64.51% <64.51%> (ø)
pkg/api-server/server.go	82.09% <100.00%> (+0.31%)	⬆️
pkg/core/resources/model/rest/resource.go	67.94% <0.00%> (-1.29%)	⬇️
pkg/xds/generator/direct_access_proxy_generator.go	82.75% <0.00%> (-1.15%)	⬇️
pkg/dns/vips_allocator.go	74.00% <0.00%> (+1.33%)	⬆️
pkg/core/resources/manager/cache.go	88.31% <0.00%> (+2.59%)	⬆️
pkg/core/bootstrap/autoconfig.go	54.46% <0.00%> (+8.03%)	⬆️
pkg/plugins/runtime/gateway/route/sorter.go	71.79% <0.00%> (+10.25%)	⬆️

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