feat: add service discovery for kong admin service

[pmalek]

What this PR does / why we need it:

This PR introduces kong Admin API service discovery via the --kong-admin-svc flag (to be discussed).

When that flag is specified (as namespace/service_name) KIC will watch EndpointSlices for that Service and add (and remove when the endpoint gets removed) a kong client for each endpoint for that Service.

This in its current form does not take into account the fact that gateway should be made ready only when its config is applied and ready (Admin API readines != Gateway Proxy readiness). We don't yet have a mechanism to be sure that a config has been applied (it can come in one of the upcoming Gateway releases) but we could implement a mechanism to make Gateway Proxy service marked as ready after the config has been sent.

#3499 covers whether we'd like to introduce said mechanism.

Which issue this PR fixes:

Fixes #3400

Special notes for your reviewer:

This is not yet 100% ready but ready for review.

The multi gw manifests that are also slightly changed in this PR are almost working, that is: they deploy KIC 2.8.1 which doesn't yet support --kong-admin-svc flag and since --kong-admin-url has a default of []string{"http://localhost:8001"} then it uses that but since with those manifests we deploy 2 separate deployments this address cannot be reached and KIC fails.

PR Readiness Checklist:

Complete these before marking the PR as ready to review:

• the CHANGELOG.md release notes have been updated to reflect any significant (and particularly user-facing) changes introduced by this PR

[codecov]

Codecov Report

Base: 72.9% // Head: 72.3% // Decreases project coverage by -0.6% ⚠️

Coverage data is based on head (9a400cf) compared to base (f6611e3).
Patch coverage: 48.3% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #3421     +/-   ##
=======================================
- Coverage   72.9%   72.3%   -0.6%     
=======================================
  Files        119     120      +1     
  Lines      14099   14318    +219     
=======================================
+ Hits       10283   10362     +79     
- Misses      3158    3289    +131     
- Partials     658     667      +9     

Impacted Files	Coverage Δ
...ntrollers/configuration/kongadminapi_controller.go	0.0% <0.0%> (ø)
internal/dataplane/synchronizer.go	90.3% <0.0%> (-3.4%)	⬇️
internal/manager/utils/kongconfig/root.go	49.5% <0.0%> (ø)
internal/manager/config_validation.go	86.7% <25.0%> (-3.9%)	⬇️
internal/manager/setup.go	47.6% <40.0%> (-3.3%)	⬇️
internal/adminapi/client.go	56.0% <50.0%> (+4.3%)	⬆️
internal/adminapi/kong.go	61.7% <72.7%> (+0.6%)	⬆️
internal/dataplane/kong_client.go	85.1% <94.3%> (+1.6%)	⬆️
internal/manager/config.go	94.3% <100.0%> (+0.1%)	⬆️
internal/manager/controllerdef.go	98.7% <100.0%> (+<0.1%)	⬆️
... and 6 more

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[pmalek]

Here's an e2e targeted pipeline that passed: https://github.com/Kong/kubernetes-ingress-controller/actions/runs/4128494573/jobs/7133003972#step:10:374

[czeslavo]

🚢

[pmalek]

e2e targeted passed: https://github.com/Kong/kubernetes-ingress-controller/actions/runs/4134273470/jobs/7145211626#step:10:492