feat: support flamegraph of kernel stack collection for pod/node

[ljluestc]

feat: support flamegraph of kernel stack collection for pod/node

• Add FlameAggregator sink to collect and aggregate kernel stacks
• Expose HTTP endpoint /flamegraph/collapsed for querying collapsed stacks
• Support node and pod scoped stack aggregation
• Add event type filtering and reset functionality
• Include unit tests

Fixes #137

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