[core][V1] pluggable scheduler

[joerunde]

While we wait for the scheduler interface overhaul on #12544, this PR aims to only make the scheduler for the V1 engine pluggable. This allows the --scheduler-cls engine arg to continue to function the same way it does in V0.

We require this for the vllm-spyre plugin, which overrides the scheduler to add additional constraints based on hardware limitations. If the intent is to go a different direction and not allow the scheduler class to be overridden at runtime for V1, we'd like to know that as well so we can figure out how best to integrate our additional constraints.

FIX vllm-project/vllm-spyre#15

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[comaniac]

I'm ok with this add-on, but prefer to be transparent about the compatibility.

[comaniac]

LGTM