[spec decode] [4/N] Move update_flash_attn_metadata to attn backend

[SolitaryThinker]

for #7000
FIX #xxxx (link existing issues this PR will resolve)

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[github-actions]

👋 Hi! Thank you for contributing to the vLLM project.
Just a reminder: PRs would not trigger full CI run by default. Instead, it would only run fastcheck CI which consists a small and essential subset of CI tests to quickly catch errors. You can run other CI tests on top of default ones by unblocking the steps in your fast-check build on Buildkite UI.

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🚀

[SolitaryThinker]

/ready

[comaniac]

Nice refactoring!

[SolitaryThinker]

Also going to add a comment explaining that most of the functionality is still in custom_ops and not refactored here yet.

[comaniac]

LGTM. The failed tests are likely due to missing API in mocked attention metadata. Should be good once added