[plugin] move custom executor to plugins

[youkaichao]

move the custom executor introduced in #6557 to plugin system, to reduce the code complexity of the main codebase.

[github-actions]

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[WoosukKwon]

Thanks for the PR! However, I'd like to push back this PR and this line of effort:

1. I think the executors are not the source of code complexity. While supporting different hardware backends does increase complexity, this is orthogonal to the executor abstraction. The complexity comes from the fact that the shared system components like scheduler should work with all different backends, which is still the case even if we make the executors pluggable.
2. Different hardware backends touch more than the executor/worker/model runner. They often touch models, weight loading utilities, device communicators, etc. Therefore, separating out them from vllm main repo may not remove the complexity and instead add another complexity of versioning.
3. I feel this is a bit too early. Can we focus on other refactoring/simplification first and get back to this topic? I think there are higher priority items that might affect the APIs and eventually this PR.

[mergify]

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

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