feat(multikueue): implement KEP-9270 stepSize for Incremental Dispatcher

[Mostafahassen1]

What type of PR is this?

/kind feature
/area multikueue

What this PR does / why we need it:

This PR implements the Alpha phase of KEP-9270,

The MultiKueue Incremental Dispatcher previously had a hardcoded
batch size of 3 when nominating worker clusters. This is too slow
for administrators managing large fleets and is not tunable without
a code change.

This PR makes the batch size configurable via a new stepSize field
inside a new IncrementalDispatcherConfig struct on the MultiKueue
configuration API. The field is guarded by the new
MultiKueueIncrementalDispatcherConfig feature gate (Alpha, disabled
by default), so existing behavior is completely unchanged unless the
gate is explicitly enabled.

Changes included:

• Feature gate: MultiKueueIncrementalDispatcherConfig registered
  at Alpha (false by default) in pkg/features/kube_features.go

• API types (v1beta1 + v1beta2): New IncrementalDispatcherConfig
  struct with a StepSize *int32 field (kubebuilder default: 3,
  minimum: 1) added to the MultiKueue struct

• Generated code: zz_generated.deepcopy.go and
  zz_generated.conversion.go updated for the new type in both
  API versions

• Defaults: SetDefaults_Configuration defaults StepSize to 3
  in both v1beta1 and v1beta2

• Dispatcher logic: IncrementalDispatcherReconciler now accepts
  cfg *Configuration and reads StepSize through a stepSize()
  helper that falls back to 3 when the feature gate is off or the
  field is unset

• Tests: New test cases for stepSize=2 first batch, stepSize=2
  second batch after round expiry, and feature-gate-disabled fallback

Which issue(s) this PR fixes:

Fixes #9270

Special notes for your reviewer:

• The KEP document was approved in KEP-9270: Introduce configuration for the MultiKueue Incremental Dispatcher #10877. This PR is the follow-up
  implementation for the Alpha milestone.

• The feature gate is off by default. No existing behavior changes
  unless MultiKueueIncrementalDispatcherConfig: true is set in
  featureGates.

• The IncrementalDispatcherConfig struct is intentionally kept as its
  own type (not inlined) to allow future fields (e.g. timeout,
  stepSizeAsPercent) to be added without breaking API compatibility,
  as described in the KEP's "Possible Follow-ups" section.

[k8s-ci-robot]

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[Mostafahassen1]

/retest

[Mostafahassen1]

/retest

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[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: Mostafahassen1
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