[Frontend] split run_server into build_server and run_server

[dtrifiro]

This aims to split run_server into two: one part responsible for building the uvicorn.Server and another responsible for running it.

With this:

• run_server() is now a coroutine, which can be run using asyncio.run() or just awaited when running in an async context
• build_server() can now take optional kwargs, which are passed to uvicorn and allow for further customization of the server

[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.

Once the PR is approved and ready to go, please make sure to run full CI as it is required to merge (or just use auto-merge).

To run full CI, you can do one of these:

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

🚀

[dtrifiro]

CI failures look unrelated?

[DarkLight1337]

Thanks for the cleanup!