[V1] Scheduler Refactoring [2/N] - Introduce CommonSchedulerStates

[WoosukKwon]

This PR introduces the CommonSchedulerStates class for the common states that every scheduler will have.
Currently, it only has two states: finished_req_ids (which is part of the scheduler interface in a broad sense) and cached_reqs_data (which is just an optimization of CachedRequestDdata. This PR is just moving these two states to a separate file, which should not affect functionality or performance.

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[WoosukKwon]

cc @comaniac

[WoosukKwon]

Regarding #14731 (comment)
I will do it in the next PR

[comaniac]

While I'm not sure if the current scope of CommonSchedulerStates is general enough, we don't have other schedulers to verify it atm, so I feel it's ok for now. We could revisit/refactor it to adjust the scope later when needed.

[comaniac]

Should this be a dataclass?

[mergify]

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

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

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