Add percentage-based offline instance thresholds for maintenance mode

[LZD-PratyushBhatt]

Currently maintenance mode entry/exit thresholds (MAX_OFFLINE_INSTANCES_ALLOWED, NUM_OFFLINE_INSTANCES_FOR_AUTO_EXIT) only support absolute counts, requiring operators to re-tune them as clusters scale. This adds percentage-based alternatives that adapt automatically to cluster size.

New ClusterConfig properties:

• MAX_OFFLINE_INSTANCES_ALLOWED_PERCENTAGE (0-100)
• NUM_OFFLINE_INSTANCES_FOR_AUTO_EXIT_PERCENTAGE (0-100)

When both absolute and percentage thresholds are configured, the stricter (lower effective count) value wins. Percentage is computed against total routable instances using integer truncation (conservative rounding).

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