[Config][Disaggregated] Add timeout configuration for the torch.store and add KVTransferConfig.kv_connector_extra_config

[hasB4K]

Hello,

Adding a kv_connector_extra_config will help custom connectors to use it to hack around if they need.
I'm using it within the SimpleBuffer/PyNcclPipe to set the torch.distributed Store's timeout (in the example, I set the timeout for 1 hour):

--kv-transfer-config '{
   "kv_connector": "PyNcclConnector",
   "kv_role": "kv_consumer",
   "kv_rank": 1,
   "kv_parallel_size": 2,
   "kv_buffer_size": 5e9,
   "kv_ip": "localhost",
   "kv_port": 12345,
   "kv_connector_extra_config": {"store_timeout": 3600}
}'

Issue like this: #10502 (comment) torch.distributed.DistStoreError: wait timeout after 300000ms, keys: /send_to/0/4
Would happen less frequently, since it only happen when you have no requests.

I think it's not safe to increase it too much, but I have been able to increase this safely to more than 1 hour. It can be useful for dev purposes, or to reduce the need of a frequent heartbeat.

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[KuntaiDu]

Some small naming changes and other parts LGTM. Feel free to suggest other naming.

[KuntaiDu]

LGTM!