Add the requirement of arctic-inference which speculative decoding with suffix_decode

[frankie-ys]

What this PR does / why we need it?

Does this PR introduce any user-facing change?

How was this patch tested?

• vLLM version: v0.12.0
• vLLM main: vllm-project/vllm@ad32e3e

[gemini-code-assist]

Code Review

This pull request adds arctic-inference==0.1.1 as a new dependency. However, this change is incomplete as the new dependency has only been added to requirements.txt and is missing from pyproject.toml. This inconsistency can lead to build failures and should be addressed by adding the dependency to pyproject.toml as well.

[github-actions]

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