Add webhook framework

[kerthcet]

Pull Request Description

• add the webhook for validations, which uses an internal cert.
• add the integration test framework and add a simple example
• Update the update-codegen.sh

Related Issues

xref: https://github.com/aibrix/aibrix/issues/710

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• [ ] PR title includes appropriate prefix(es)
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[kerthcet]

I hope to push the changes commit by commit, however, the dependency is really coupled, so ...

[kerthcet]

I think the doc build error is not related because it's a private repo at this moment.

[kerthcet]

Failed by TestBaseModelInferenceFailures/Invalid_API_Key, rerun.

[kerthcet]

Could you take a look @Jeffwan This is the first step to introduce the webhook framework. Thanks!

[Jeffwan]

I was busy on the open source stuff yesterday. I will spend some time on the review todya

[Jeffwan]

doc build failure is expected, I switch to free version and it only supports public repo. It will come back soon

[Jeffwan]

@kerthcet #740 and #741 have been merged. We can revisit this one now

[kerthcet]

Migrate to https://github.com/vllm-project/aibrix/pull/748 as a forked repo's PR. I'll close this one.