Add and use pod labels to avoid having to query all pods from kubernetes

[otrosien]

One-line summary

Implements pod labeling optimization for Kubernetes Elasticsearch operator to avoid querying all pods.

Issues

• Fixes Add and use pod labels to avoid having to query all pods from kubernetes #461

Key Changes:

• Added migration logic in setupInformers() to ensure all existing EDS pods have the es-operator-dataset label
• Modified collectResources() to use label selectors when fetching pods for each EDS
• Updated templateInjectLabels() to always set labels (removed conditional check)
• Added comprehensive logging and annotation-based migration tracking
• Created E2E test to verify pod label migration functionality
• Introduced new annotation es-operator.zalando.org/pod-label-migration-complete to track migration status

Impact:

• Core change to pod discovery mechanism that could affect all EDS resources
• Migration runs on every operator startup (with annotation-based skip logic)
• Performance improvement expected from targeted pod queries vs. listing all pods

Types of Changes

• New feature (non-breaking change which adds functionality)

Review

List of tasks the reviewer must do to review the PR

• Tests
• Documentation
• CHANGELOG

Deployment Notes

• Document migration steps

[zalando-compr-opensource]

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