v1: Support KV events from connectors

[orozery]

This PR adds a new scheduler-side connector API to collect KV cache events.
Additionally, we add a generic metadata field to KVCacheEvent to allow custom metadata on KV events.

Part of the work described in RFC #19854

[gemini-code-assist]

Summary of Changes

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This pull request enhances the KV cache event system by extending the connector API to allow connectors to emit KV cache events. It also introduces a flexible metadata field for KVCacheEvent objects and integrates the collection of these new connector-emitted events into the scheduler's event publishing mechanism.

Highlights

• KVCacheEvent Metadata: Introduced a generic metadata: Optional[dict[str, Any]] field to the KVCacheEvent base class, allowing custom data to be associated with KV cache events. The KVCacheEvent dataclass was also updated to make its fields keyword-only.
• New KV Event API for Connectors: Added a new take_events() method to the KVConnector base class. This method provides an API for connectors to expose KV cache events they collect, returning an optional list of KVCacheEvent objects.
• Scheduler Event Aggregation: Updated the scheduler's schedule method to collect and publish KV cache events. It now aggregates events from both the existing kv_cache_manager and the newly introduced KVConnector.take_events() method, ensuring all relevant events are published.

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[gemini-code-assist]

Code Review

This PR introduces a new scheduler-side connector API to collect KV cache events, along with a generic metadata field for KVCacheEvent. The changes look good, with a suggestion for optimizing list concatenation in the scheduler and a few minor suggestions for improved code clarity.

[github-actions]

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