[V1][Metrics] add support for kv event publishing

[alec-flowers]

RFC: KVBlocks and Metrics Publishing In Inference Frameworks

• Added KVCacheEvent, BlockStored, BlockRemoved, and AllBlocksCleared msgspec classes
• Created a queue in the BlockPool and write these events in the appropriate functions
• Bubble the events up to the scheduler where they are appended to EngineCoreOutputs
• Add kv_cach_events to EngineCoreOutputs
• Wrote unit tests at the BlockManager level to test basic functionality and at the EngineCore level testing correct propagation and serializing over zmq.

API

• add enable_kv_cache_events to engineArgs ~- add external_stat_loggers field to AsyncLLM API~ Covered by https://github.com/vllm-project/vllm/pull/14661

With https://github.com/vllm-project/vllm/pull/14661 and this PR a 3rd party can write a custom stat logger to consume both engine Stats and Events and publish them elsewhere.