clean memory scaler object when pa crd is deleted

[kr11]

Pull Request Description

[Please provide a clear and concise description of your changes here]

This PR addresses an issue where deleting a PA object does not remove the associated scaler object from memory, potentially leading to increased memory usage and bugs upon PA recreation.

Changes Introduced:

• Added cleaning logic that triggers when a PA is detected as deleted.
• Enhanced the NamespaceMetric structure to include PA information. Previously, it only contained TargetRef details. Now, it also records the PA's namespace and name, which are accessible post-deletion.

logs when creating pa:

I1211 16:07:48.072607   55435 podautoscaler_controller.go:658] "New scaler added to AutoscalerMap" metricKey="default/mock-llama2-7b" type="KPA" spec={"scaleTargetRef":{"kind":"Deployment","name":"mock-llama2-7b","apiVersion":"apps/v1"},"minReplicas":0,"maxReplicas":10,"metricsSources":[{"metricSourceType":"pod","protocolType":"http","path":"metrics","port":"8000","targetMetric":"avg_prompt_throughput_toks_per_s","targetValue":"60"}],"scalingStrategy":"KPA"}

logs when deleting pa:

I1211 16:08:02.635597   55435 podautoscaler_controller.go:135] "Delete scaler" PaName="podautoscaler-mock-llama2-7b" PaNamespace="default" TargetRefNamespace="default" TargetRefName="podautoscaler-mock-llama2-7b"
I1211 16:08:03.391976   55435 podautoscaler_controller.go:167] PodAutoscaler resource not found. Clean scaler object in memory since object default/podautoscaler-mock-llama2-7b must have been deleted

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