Fix defaulting and validation around the two modes for function scaling ( default vs. external scaler )

[kwiatekus]

Description

Fix the validation, defaulting logic so that it conforms with the following case description.
Write tests that validates the scenarios.

when
user defines spec.scaleConfig.minReplicas and spec.scaleConfig.maxReplicas (higher than min)
than
default ( serverless managed Horizontal Pod Autoscaler kicks in to scale the function)

when
user defines spec.scaleConfig.minReplicas and spec.scaleConfig.maxReplicas (values equal or max lower than min)
than
validation webhook rejects such configuration

when
user leaves spec.scaleConfig empty
then
the default HPA scaler would not kick in

when
user defines spec.replicas with X
then
the deployment has X replicas

when
user leaves both spec.replicas and spec.scaleConfig empty
then
the defaulting webhook should fill in spec.replicas=1

Reason
Function CR support scale subresource that would enable scaling functions via external scalers.
Additionally it still offers the way to configure default scaling ( via serverless managed HPA ) based on cpu consumption
Those configuration modes should be tested

Related
#15291
#15292

[kwiatekus]

Investigate whether function controller managed HPA scaling the function would simplify the logic ( p 3 ).

[dbadura]

I would add case when user to max or min replicas.
In this case we should reject such configuration.

[kwiatekus]

Works like described since #15576