Support selector for InferencePool selection in InferenceObjective

[pierDipi]

What type of PR is this?
/kind feature

What this PR does / why we need it:

Currently, InferenceObjective requires a direct reference to a single InferencePool via poolRef. In multi-pool environments, administrators must create duplicate InferenceObjective resources for each pool, leading to:

• Configuration sprawl and maintenance burden
• Potential for configuration drift between pools
• Difficulty applying consistent priority policies across pool group

  spec:                                                                                                                                                                                                                                                             
    priority: -10                                                                                                                                                                                                                                                    
    poolSelector:             # mutually exclusive with poolRef, within InferenceObjective namespace                                                                                                                                                                                                                                                   
      matchLabels:                                                                                                                                                                                                                                                  
        gradution: beta                                                                                                                                                                                                                                            
      matchExpressions:                                                                                                                                                                                                                                            
        - key: plan                                                                                                                                                                                                                                               
          operator: In                                                                                                                                                                                                                                              
          values: [plus, pro]

Which issue(s) this PR fixes:

Fixes #2202

Does this PR introduce a user-facing change?:

Support selector for `InferencePool` selection in `InferenceObjective`

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: pierDipi
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[k8s-ci-robot]

Hi @pierDipi. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[nirrozenbaum]

@pierDipi what would happen if the label selector of InfObj is:

labels:
  env: prod

and then we add this label to an existing pool?
I see no handling of pool changes that trigger a match. am I missing something?

[kfswain]

I'd like to hold off on this if we can.

This doc proposes changes to InferenceObjective that increases its utility beyond simple priority. And so some of the semantics may become pool specific, rather than reusable.
https://docs.google.com/document/d/1JxzJc8gNv2wKK5-a8ohb0btn78ymVKw9XMIb4-S-ncA/edit?tab=t.0