[V0][Minor] Support custom add a new kv connector without modify vllm code

[maobaolong]

The motivation of this PR aimed to make it easy to use a new kv connector implementation of third-party without modify vllm code.

In the other hand, vllm should not responsible for each custom kv connector implementations.

[github-actions]

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[github-actions]

This pull request has been automatically marked as stale because it has not had any activity within 90 days. It will be automatically closed if no further activity occurs within 30 days. Leave a comment if you feel this pull request should remain open. Thank you!

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @maobaolong.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[maobaolong]

As this has been merged by #18142 , so I closed this PR, and thanks @sdavidbd for your PR.