[Core] get_kv_cache_from_block within core ONLY

[KrishnaM251]

See #6808 for full details

[v0.3.1]

• implemented get_kv_from_block at the block_manager_v2.py level
• get_kv_from_block simply calls lower level methods from within core to obtain the kv_cache without going to the worker
  • used the prepare_worker_input from worker.py as inspiration

[github-actions]

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