[Misc] feat: Support external kv_connector for vllm v1

[dtcccc]

See vllm-project/vllm#24718

Support mooncake transfer engine in vllm v1 by using dynamic external kv_connector.

Example usage:

Adding the following params to vllm command:
Prefill: --kv-transfer-config '{"kv_connector":"MooncakeConnector","kv_role":"kv_producer", "kv_connector_module_path":"mooncake.mooncake_connector_v1"}'
Decode: --kv-transfer-config '{"kv_connector":"MooncakeConnector","kv_role":"kv_consumer", "kv_connector_module_path":"mooncake.mooncake_connector_v1"}'
Proxy: Running tests/v1/kv_connector/nixl_integration/toy_proxy_server.py

[ShangmingCai]

Thx for the contribution! LGTM. Before we merge it, can you add a demo or a README on how to use it?

[dtcccc]

Thx for the contribution! LGTM. Before we merge it, can you add a demo or a README on how to use it?

Added a demo in README.

[stmatengss]

Could you integrate a proxy server from vllm rep？ So we don't rely on vllm proxy server

[ShangmingCai]

LGTM. Optimizations and proxy can be placed in the future PR. Thx for the contribution.