feat: add Config.Validate() to allow validation of configuration options before starting the manager

[pmalek]

What this PR does / why we need it:

This PR adds Config.Validate() to allow validation of configuration options before starting the manager.

This will also set the stage to allow stopping the manager boot up process when mutually exclusive options are being provided (e.g. as in single controller deployments kong-admin-url and potential future Admin API service name specified for service discovery).

Additionally this performs some minor refactor around config and passing down the configuration options to downstream consumers of said options. It also stores the flagset in the Config so that we can use c.flagSet.Changed() to verify if the flag has been provided by the user ( in case the default flag value is a non empty/default value).

[codecov]

Codecov Report

Base: 74.0% // Head: 73.8% // Decreases project coverage by -0.3% ⚠️

Coverage data is based on head (82f45e5) compared to base (e9dd1b5).
Patch coverage: 39.5% of modified lines in pull request are covered.

❗ Current head 82f45e5 differs from pull request most recent head 1d5a344. Consider uploading reports for the commit 1d5a344 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #3405     +/-   ##
=======================================
- Coverage   74.0%   73.8%   -0.3%     
=======================================
  Files        113     114      +1     
  Lines      13892   13928     +36     
=======================================
- Hits       10291   10287      -4     
- Misses      2954    2985     +31     
- Partials     647     656      +9     

Impacted Files	Coverage Δ
internal/cmd/rootcmd/run.go	21.0% <0.0%> (-4.0%)	⬇️
internal/cmd/rootcmd/rootcmd.go	13.8% <16.1%> (+13.8%)	⬆️
internal/manager/setup.go	55.6% <18.5%> (-5.1%)	⬇️
internal/manager/run.go	44.7% <26.6%> (-9.2%)	⬇️
internal/manager/flagnamespacedname.go	85.7% <85.7%> (ø)
internal/manager/config.go	90.7% <100.0%> (+0.5%)	⬆️
internal/manager/controllerdef.go	98.7% <100.0%> (ø)
internal/manager/feature_gates.go	100.0% <100.0%> (ø)
...nternal/controllers/gateway/udproute_controller.go	69.5% <0.0%> (-4.3%)	⬇️
...ternal/controllers/gateway/httproute_controller.go	58.5% <0.0%> (-1.6%)	⬇️
... and 5 more

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[czeslavo]

I like the direction of making the validation run as soon as possible, not leaving the responsibility to the routines that accept the Config. It's gonna be less error-prone and will save us from scattering validation code over places that should not be responsible for that.