fix(config): correctly check types.NamespacedName flags are set

[czeslavo]

What this PR does / why we need it:

Makes every types.NamespacedName flag in the manager.Config a mo.Option which directly express the fact that these flags are optional. mo.Option have handy methods that allow checking whether a value was set or not (IsPresent() bool, IsAbsent() bool or Get() (value T, ok bool)).

We had a few cases in which we were checking that types.NamespacedName is not empty using its String() method which is implemented like so:

func (n NamespacedName) String() string {
	return n.Namespace + string(Separator) + n.Name
}

Even with no Namespace and Name set, String() != "" checks always evaluated to true due to the fixed separator.

Which issue this PR fixes:

Special notes for your reviewer:

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: 72.6% // Head: 72.7% // Increases project coverage by +0.0% 🎉

Coverage data is based on head (5c46810) compared to base (9c97200).
Patch coverage: 74.5% of modified lines in pull request are covered.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #3602   +/-   ##
=====================================
  Coverage   72.6%   72.7%           
=====================================
  Files        131     131           
  Lines      15675   15694   +19     
=====================================
+ Hits       11385   11411   +26     
+ Misses      3535    3529    -6     
+ Partials     755     754    -1     

Impacted Files	Coverage Δ
internal/manager/run.go	37.9% <0.0%> (ø)
internal/manager/setup.go	41.2% <23.0%> (-0.8%)	⬇️
internal/manager/config.go	94.4% <100.0%> (ø)
internal/manager/config_validation.go	88.3% <100.0%> (+0.9%)	⬆️
internal/manager/controllerdef.go	98.8% <100.0%> (ø)
internal/manager/validated.go	100.0% <100.0%> (+13.0%)	⬆️
...nternal/controllers/gateway/tlsroute_controller.go	71.4% <0.0%> (-2.1%)	⬇️
...ternal/controllers/gateway/grpcroute_controller.go	69.7% <0.0%> (ø)
internal/dataplane/parser/parser.go	90.7% <0.0%> (+1.6%)	⬆️
...nternal/controllers/gateway/tcproute_controller.go	72.0% <0.0%> (+2.0%)	⬆️
... and 1 more

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[randmonkey]

overall LGTM, some minor comments.

[czeslavo]

@randmonkey @pmalek PTAL again, I believe all of your suggestions have been addressed: 3dec228.

[pmalek]

I believe this should do it. Just one nit is that we might consider adding a type alias for mo.Option[types.NamespacedName] (this can be done in a separate PR though).

[czeslavo]

I believe this should do it. Just one nit is that we might consider adding a type alias for mo.Option[types.NamespacedName] (this can be done in a separate PR though).

Let's do it straight away: 5c46810