[Core] add xgrammar as guided generation provider

[joennlae]

Essentially a cleaned-up version of this pr:
#10785

Especially since outlines is relatively slow and the new version is tough
to integrate as they do not focus on being pickleable, which is a crucial
feature for us using the multiprocessing engine: dottxt-ai/outlines-core#99

I assume more and more will change over to xgrammar.

This is a minimum implementation.

https://arxiv.org/pdf/2411.15100

[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 starts running only a small and essential subset of CI tests to quickly catch errors. You can run other CI tests on top of those by going to your fastcheck build on Buildkite UI (linked in the PR checks section) and unblock them. If you do not have permission to unblock, ping simon-mo or khluu to add you in our Buildkite org.

Once the PR is approved and ready to go, your PR reviewer(s) can run CI to test the changes comprehensively before merging.

To run CI, PR reviewers can do one of these:

• Add ready label to the PR
• Enable auto-merge.

🚀

[mergify]

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

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

[mgoin]

Hey @joennlae we landed initial support here #10785. Let me know if something wasn't included that you need. We will be following up with more performance optimizations soon.

[joennlae]

@mgoin All good, perfect :-) I think there is just some more cleanup here. But I close it for now as the support is what I care about :-)