[Core] Make scheduling policy settable via EngineArgs

[schoennenbeck]

This PR makes the policy for the scheduler settable via the EngineArgs.

So far the new priority-policy could only be activated in an LLMEngine by altering the scheduler after creation. This makes the policy accessible at the time of engine creation.

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[github-actions]

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[schoennenbeck]

Questions for reviewers:

• I chose scheduling_policy over policy because in EngineArgs just naming it policy it feels unclear what this actually does. Thoughts?
• I opted for stricter typing with Literal but could also change it to just str if you prefer.

[DarkLight1337]

Questions for reviewers:

• I chose scheduling_policy over policy because in EngineArgs just naming it policy it feels unclear what this actually does. Thoughts?
• I opted for stricter typing with Literal but could also change it to just str if you prefer.

Your choices sound good to me.

Can you clarify what "fcfs" means in the help text? I guess it means first-come-first-served but it's better to be more explicit in the documentation.

[DarkLight1337]

Thanks for your effort!