ForwardContext does not offer a stable API

[liranschour]

Add get_kv_cache_attn_layers() to ForwardContext to provide a more stable API.

Connector implementations should use this API to retrieve KV cache attention layers as follow:
attn_layers = forward_context.get_kv_cache_attn_layers()

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[tlrmchlsmth]

Check out #17222, which similarly improves the interfaces to flash-attention and LMCache could use that interface instead of what it does here https://github.com/LMCache/LMCache/blob/048001e9df18e929f275a86d3bc1bae2c9fde189/lmcache/integration/vllm/vllm_v1_adapter.py#L344-L355

[tlrmchlsmth]

What is this for?

[liranschour]

@tlrmchlsmth OK will use VllmConfig.get_layers_from_vllm_config() inside LMCache and close this PR.